On the hot, dusty road from the airport, she had watched rickshaws surge past women sweeping the streets, bone-thin in their bright saris. Now, in a language she barely understood, unfamiliar aunts and uncles lamented her fate: to be forced to leave the United States, her home since kindergarten, because the F.B.I. had mysteriously identified her as a potential suicide bomber.

"I feel like I'm on a different planet," the girl, Tashnuba Hayder, said. "It just hit me. How everything happened - it's like, 'Oh, my God.' "

The story of how it happened - how Tashnuba, the pious, headstrong daughter of Muslim immigrants living in a neighborhood of tidy lawns and American flags, was labeled an imminent threat to national security - is still shrouded in government secrecy. After nearly seven weeks in detention, she was released in May on the condition that she leave the country immediately. Only immigration charges were brought against her and another 16-year-old New York girl, who was detained and released. Federal officials will not discuss the matter.

But as the first terror investigation in the United States known to involve minors, the case reveals how deeply concerned the government is that a teenager might become a terrorist, and the lengths to which federal agents will go if they get even a whiff of that possibility. And it has drawn widespread attention, stoking the debate over the right balance between government vigilance and the protection of individual freedoms.

It is not known what prompted the authorities to investigate Tashnuba, who says the accusations are false. But in a series of interviews - her first - she said the government had apparently discovered her visits to an Internet chat room where she took notes on sermons by a charismatic Islamic cleric in London, a sheik who has long been accused of encouraging suicide bombings.

An F.B.I. agent, posing as a youth counselor, first confronted Tashnuba in her bedroom, going through her school papers and questioning everything from her views on jihad to her posterless walls, she said. Sent to a center for delinquents in Pennsylvania, Tashnuba said she was interrogated without a lawyer or parent present, about her beliefs and those of her friends, mainly American girls she had met at city mosques.

As suicide bombings mount overseas, with teenage girls among the perpetrators, there is no doubt that the government's intelligence efforts are spurred by legitimate fears. The agent leading this investigation was a Muslim woman born in Britain who has voiced strong concern about radical clerics' influence on young immigrants there. And in Tashnuba, who wore a veil and talks of an ideal Islamic state, she met unsettling opinions and teenage defiance.

But Tashnuba says that she opposes suicide bombing, that her interest in the cleric was casual, and that the government treated her like a criminal simply for exercising the freedoms of speech and religion that America had taught her.

As she tells it, F.B.I. agents tried to twist mundane details of her life to fit the profile of a terrorist recruit, and when they could not make a case, covered their tracks by getting her out of the country. In fact, the court order of "voluntary departure" that let her leave requires a finding that the person is not deportable for endangering national security.

Tashnuba said she believed she was singled out precisely because she is a noncitizen - allowing investigators to invoke immigration law, bypassing the familiar limits of criminal and juvenile proceedings.

"That gave them the green light to get me out of my family," Tashnuba said during her long journey with her mother and siblings to this teeming city where she was born.

This account is, in large part, her version of events. Some of it is supported by documents and other interviews, but it cannot all be corroborated because a court has sealed the case record at the F.B.I.'s request and barred participants from disclosing government information. The government has declined repeated requests to present its side.

'Alarm Bells' for F.B.I.

Two former F.B.I. agents, presented with the known details of the case, declined to discuss it specifically, but spoke of the pressures and practices that shape such investigations today.

Pasquale J. D'Amuro, who headed the New York F.B.I. office until April, said that since 9/11, agents have had to err on the side of suspicion. More potential threats are being reported, he said, and every one must be thoroughly investigated through whatever avenues are legally available, including enlisting immigration authorities as soon as a noncitizen is under scrutiny.

"The alarm bells are going off," said Mr. D'Amuro, now the chief executive of Giuliani Security and Safety, a consulting company. "And we have each and every time to run those threats to the ground, whether it ends up to be a bogus threat or proceeds to some type of prosecutorial action."

Some cases are never resolved, he added. Even when suspicions prove unfounded, he said, any visa violations are already in the hands of immigration authorities, who have to bring them "to some type of closure."

But Mike German, who left the bureau a year ago after a long career chasing homegrown terror suspects, said that the agency's new emphasis on collecting intelligence rather than criminal evidence has opened the door to more investigations that go "in the wrong direction."

"If all these chat rooms are being monitored, and we're running down all these people because of what they're saying in chat rooms, then these are resources we're not using on real threats," said Mr. German, who has publicly complained that F.B.I. management problems impeded terror investigations after 9/11.

The stress on intelligence increases the agency's demands for secrecy, to protect its sources. And secrecy, he said, leads to abuses of power.

"Perhaps the government has some incredibly incriminating piece of information and saved us from a terrible act of violence; it would make everybody feel better to know it," he said. "Conversely, if they did something wrong, the public needs to know that."

From the beginning, the government framed this case as purely an immigration matter. When a dozen federal agents plucked the girl from her home in a dawn raid on March 24, they cited only the expiration of her mother's immigration papers, telling the family that Tashnuba would probably be returned the next day.

Instead, after two weeks of frantic inquiries by her parents, The New York Times learned that Tashnuba was one of two girls being held, officially on their parents' immigration violations, but actually for questioning by F.B.I.'s Joint Terrorism Task Force.

According to a government document provided to The Times by a federal official, the F.B.I. asserted that the girls presented "an imminent threat to the security of the United States based upon evidence that they plan to be suicide bombers." The document cited no evidence. And in background interviews, federal officials were quick to play down the case as soon as reporters called, characterizing the investigation as a pre-emptive move against potential candidates for recruitment, not the disruption of a plot.

By then agents had seized Tashnuba's diary, schoolwork and phone book - and the computer she had repeatedly tuned to sermons broadcast daily by Sheik Omar Bakri Muhammed. From her account of the agents' questions, and comments by a government official who reviewed a report about the F.B.I.'s grounds for suspicion, it appears that Tashnuba's interest in the speeches became the lens that colored everything else about her life.

Veering between "nice and awful," she said, up to three agents at a time pressed her about possible terrorist ties among her friends, and what they saw as suspicious tendencies in her schoolwork, like class notes about suicide. She said they even criticized the austere décor of the bedroom she shared with her 10-year-old sister.

"The F.B.I. tried to say I didn't have a life - like, I wasn't the typical teenager," Tashnuba said bitterly, fingering her long Muslim dress. "They thought I was anti-American because I didn't want to compromise, but in my high-school ethics class we had Communists, Democrats, Republicans, Gothics - all types. In all our classes, we were told, 'You speak up, you give your opinion, and you defend it.' "

The lesson backfired, she said, when she found herself stubbornly debating the Koran's definitions of jihad with the lead F.B.I. agent: Foria Younis, a Muslim immigrant of a much more secular stripe.

"It got personal," Tashnuba said.

Behind the Veil

She is a made-in-Queens mix of devotion and defiance, this slim, dark-eyed adolescent who arrived in Astoria with her family at age 5. In her round schoolgirl handwriting, she has compiled lists of favorite prayers and pious resolutions, like "practice lowering gaze to fullest" and "be xtra nice to parents." But when she recalls how F.B.I. agents questioned her religious lifestyle, her voice drips typical teenage scorn: "Like, I'm supposed to live for you guys?"

From childhood, Tashnuba embraced religion with a kind of rebellion. By 10 she was praying five times a day - and reproaching her more secular father, a salesman of cheap watches. At 12, Tashnuba even explored Christianity. But at 14, she adopted a full Islamic veil.

In part, she was emulating her closest friend, Shahela, an American citizen who, in an interview, described veiling as a way to oppose "the degrading treatment of women's bodies as commodities" and "to hold on to my faith after 9/11." It also provided Tashnuba a refuge from her parents' marital rifts and fragile reconciliations. Soon, the two friends were conducting religious classes for other girls at city mosques.

"This is what gives me an identity," Tashnuba said of her religion.

It also estranged her from the raunchy banter at her Manhattan high school. And when Shahela opted for accelerated home schooling, Tashnuba wanted to do likewise. Her parents resisted, and rejected her alternate escape plan: an arranged marriage to an American Muslim man from Michigan named Latif, whom Tashnuba had met only fleetingly. He was not a Bangladeshi, but a blue-eyed, 21-year-old salesman of Italian, Brazilian and German descent.

What she calls "a rough time in my life" reached a crisis last October. The family had just moved to Queens Village, leaving her friends behind. When Latif suggested an elopement to Michigan, Tashnuba impulsively agreed. A few hours from New York, they heard that her father had gone to the police, and quickly drove back. The police report would come back to haunt Tashnuba.

For now, her parents agreed to home schooling, through a correspondence course. But she still had time for PalTalk, a popular Web service where she found Sheik Omar's nightly London broadcasts carried live at 2 p.m.

"It was a casual thing," she said. "I would have it on for a few minutes, then I would be going to CVS for my mom, whatever."

Parts of the broadcasts have long alarmed counterterrorism investigators, who say the Syrian-born Sheik Omar urges young Muslim men worldwide to support the Iraq insurgency on the front line of "the global jihad," and praises the 9/11 hijackers and suicide bombings. In a chilling exchange reported by The Times of London in January, a female listener asked whether "sisters are allowed to do suicide bombings if the intentions are correct." The newspaper reported that the sheik replied: "This is no problem; there is no restriction."

But in a telephone interview, the sheik denied recruiting anyone. "Nobody said to women that they should become a suicide bomber," he said.

Tashnuba said the topic never came up while she listened. What she recalled was talk of a utopian Islamic state that would follow God's will, not human desires. "You don't pay for water in an Islamic state, you don't pay for transport," she said. "There are certain rights that can't be taken away."

The Student and the Agent

At 9, an age when Tashnuba was turning to prayer, Foria Younis was beating boys at soccer in a Pakistani neighborhood in east London. Now 37, this former prosecutor is a 5-foot-2, "gun-toting, door-kicking member of the F.B.I.'s counterterrorist squad" who has hunted terrorists on three continents, according to a long profile last year in The Daily Telegraph of London.

Though Ms. Younis would not agree to an interview for this article, she did not quarrel with The Telegraph's depiction.

But on March 4, when she knocked at the Hayder family's door, Ms. Younis and her partner did not reveal that they were F.B.I. agents, said Tashnuba's mother, Ishrat Jahan Hayder. They claimed to be from a youth center, following up on the police report filed five months earlier when the girl tried to elope. Mrs. Hayder readily sent the woman upstairs to her daughter's bedroom. "I trusted her," she said.

From the moment she walked in, as Tashnuba tells it, Ms. Younis started paging through her papers. "She was like, 'Can I look at this?' Not waiting for an answer."

What mainly drew the agent's eye, the girl said, were papers from an extra-help class for home-schooled girls that Tashnuba had joined to prepare for exams. On one page was a diagram highlighting the word "suicide" - her notes on a class discussion about why religions oppose it, she said.

Soon, she said, Ms. Younis was dropping comments like "So, I see you're interested in suicide," and "So, you like staying all by yourself in your room. Are you a loner?"

Tashnuba, who had many friends, was immediately nervous and defensive. "No, I'm just in my room," she said she protested. "I saw where they were going."

Three weeks later - two days after Ms. Younis wrote a secret declaration about Tashnuba, court documents show - immigration agents raided the house. As an immigration matter, that was highly unusual; there was no active proceeding against her mother or father, whose separate, long-pending applications for political asylum had lapsed without action in the late 1990's.

But Tashnuba said the agents told her, "Your mom just admitted you're not here legally and we have to take you, or else take everybody." At immigration headquarters in Manhattan, the F.B.I. was waiting, along with the other girl, Adama Bah, a native of Guinea whom Tashnuba said she knew slightly from a Manhattan mosque. Ms. Bah was of less interest to the authorities than Tashnuba, according to the government official who reviewed F.B.I. reports.

At day's end, the girls were driven to a maximum-security juvenile detention center in rural Berks County, Pa. Suddenly they were among delinquent girls accused of drug crimes and assaults. Tashnuba was required to wear a sweat suit, march at attention and submit to strip-searches, she said. And the questioning began in earnest.

"They tried to twist my mind," Tashnuba said. "They had their little tactics - start with nice questions, try to get more severe. In the end, when I did cry they were, like, mocking me."

A government psychiatrist concluded that she was neither suicidal nor homicidal, and recommended her release. But the agents, Tashnuba said, kept "trying to link me to the psychological state." They zeroed in on the single artificial rose in her bedroom (her little sister's); a psychology course (required by her correspondence program), and an essay she wrote about the Department of Homeland Security (assigned as a writing evaluation by her tutor).

The tutor, Asmaa Samad, recalled the essay as innocuous: "It said nothing derogative, nothing unpatriotic." Tashnuba said agents seized on one part. "I wrote, 'I feel like Muslims are being targeted, they're being outcasted more.'"

But instead of backing away from opinions that the agents seemed to find alarming, Tashnuba said she dug in her heels, especially on her belief in jihad. "If Islam is threatened, you have a right to fight back," Tashnuba declared, citing Koran verses.

The questioning went on, she said, from March 24 to April 7 - the day the first article about the case appeared.

As the news spread, an advocacy group arranged a lawyer for her. The Bangladeshi general consul in New York pressed the government for an explanation, and Homeland Security replied: The sole reason Tashnuba was being held was her "unlawful presence" in the United States.

The other girl was allowed to return to her East Harlem high school in early May, under strict conditions including an order not to discuss the case. But for Tashnuba, there was no prospect of release, her lawyer, Troy Mattes, said he was told.

Broke and distraught, Tashnuba's mother asked to take "voluntary departure" with her daughter, rather than fight. The government agreed, and an immigration judge issued the necessary order.

Arriving in Dhaka on May 12, Tashnuba walked into her new life and burst into tears. "I want to go back," she cried.

Her father and 14-year-old brother had stayed in hiding in New York, hoping to avoid deportation while the boy finished school. With no money for a home, Tashnuba and her mother, baby brother and little sister, Tamana, were to share an aunt's bed at her grandmother's apartment, now occupied by nine people.

For Tashnuba and Tamana, an American citizen who speaks only English, more education may be unaffordable, her mother said. Even Tashnuba's piety was challenged. Veiling is taboo among her relatives, and few Dhaka mosques allow women.

At one point in the journey, she had wished she had never gone to America, raging, "I see now you have no privacy, no liberty." But now she longed for even one more day in New York, "to say goodbye."

Fighting tears, she fell silent, staring at the shelf of souvenirs her family had sent back over the years: a big apple, a snow globe of the twin towers, a Statue of Liberty.

William K. Rashbaum, in New York, and Souad Mekhennet, in Frankfurt, contributed reporting for this article.