But Mr. Kar, 44, a naturalized American born in Iran, followed his dream where few others might have gone. In mid-May, he traveled to Iraq with an Iranian cameraman to film archaeological sites around Babylon. After a taxi they were in was stopped in Baghdad, the two men were arrested by Iraqi security forces, who found what they suspected might be bomb parts in the vehicle.

Since then, Mr. Kar has been held in what his relatives and their lawyers describe as a frightening netherworld of American military detention in Iraq - charged with no crime but nonetheless unable to gain his freedom or even tell his family where he is being held.

He is one of four men with dual American citizenship who have been detained in Iraq beginning in April, a Defense Department official said. But none of the others - all Iraqi-Americans suspected of ties to the insurgency - nor an accused Jordanian-American terrorist operative captured in a raid last year appear to have had anything like Mr. Kar's ties to the United States.

Mr. Kar, the son of an Iranian physician, came to the United States when he was 2 and was raised partly in Utah and Washington State, where he played high school football. He attended college in California, received a master's degree in technology management from Pepperdine University, worked for years in Silicon Valley and served in the United States Navy and the Naval Reserve.

Nonetheless, Mr. Kar's relatives and their lawyers said they had been utterly stymied in trying to learn his fate despite repeated inquires at the Defense Department, the Justice Department, the State Department, the allied forces in Iraq and the offices of two United States senators.

Parvin Modarress posts a photograph in her nephew's apartment. She said an F.B.I. agent had told her Mr. Kar had been "cleared

The relatives said the only detailed information they had received came from one of the F.B.I. agents who searched Mr. Kar's apartment in the Silver Lake neighborhood of Los Angeles on May 23. They said that after analyzing his personal files, computer drives and other materials, the agent, John D. Wilson, returned the seized items on June 14 and assured them that that the F.B.I. had found no reason to suspect Mr. Kar.

"He's cleared," one of Mr. Kar's aunts, Parvin Modarress of Los Angeles, quoted Mr. Wilson as saying, "They were waiting for a lie-detector machine, but they finally got it. He passed the lie-detector test."

M. Catherine Viray, a spokeswoman for the F.B.I.'s office here, said she could not comment on either the bureau's investigation of Mr. Kar or Mr. Wilson's conversations with his relatives.

A spokesman for the Defense Department, Lt. Col. John A. Skinner, said he could not confirm that Mr. Kar was being held by American forces in Iraq, citing a Pentagon policy against the disclosure of the names of detainees.

A Defense Department official who spoke on condition of anonymity because of that Pentagon policy said Mr. Kar, his cameraman, Farshid Faraji, and a taxi driver were arrested by Iraqi security forces in Baghdad on May 17, when a search of the taxi turned up "dozens" of washing machine timers - devices that Iraqi insurgents have used to make improvised explosive devices.

The three men were turned over to allied forces that the same day, the official said, and have since been treated humanely and in accordance with United States policy. All three men continue to be held in different American-run detention facilities while their cases are investigated.

"Certainly there was enough information to merit the Iraqi security forces detaining these individuals," the defense official said.

Lawyers from the American Civil Liberties Union of Southern California, who are representing Mr. Kar's relatives, said they would file a lawsuit on Wednesday in Federal District Court in Washington, accusing the government of holding Mr. Kar in violation of American and international laws and seeking his release through a writ of habeas corpus.

"Saddam Hussein has had more due process than Cyrus Kar," said Mark Rosenbaum, the lead lawyer in the case. "This is a detention policy that was drafted by Kafka."

Colonel Skinner, the Pentagon spokesman, said any American civilians detained as a possible threat to the allied forces would eventually go before a board of three American officers, who would assess their cases and decide what to do with them. He said he did not know whether there was any specific time period by which such a review would be done.

Materials taken and returned at the Los Angeles home of Cyrus Kar, a naturalized American.

"We have absolutely no desire to hold anyone longer than is necessary," Colonel Skinner said. "But you can't be wrong, either. We are talking about life-or-death issues. You have to absolutely be thorough."

Mr. Kar's sister, Anna, described her brother in a telephone interview from Nairobi as "the last person who could ever be a threat." She said her brother "really believed in Bush's foreign policy," adding, "He believed sincerely that exporting American democracy would make the world a better place."

Ms. Kar, who works for the International Committee of the Red Cross in Africa, said she had discouraged her brother from going to Iraq and was pleasantly surprised when she received a call on May 24 from a Red Cross colleague in Iraq, who said she had just seen Mr. Kar.

"I said: 'Oh, great! What a coincidence that you met him over there,' " Ms. Kar said. "Then she said, 'No, I just visited him - in detention.' "

That visit, however, was about the only hard evidence Mr. Kar's family has received about where or how he is held. He has made three brief, furtive telephone calls to his relatives in Los Angeles, but has not told them anything more than that he is being held "by the Americans" and that he fears for the fate of his cameraman, from whom he was separated.

Mr. Kar's aunt, Ms. Modarress, said she had asked him in one of the calls if he had been tortured.

"He said: 'Not now. At the beginning. Where I am now is like a country club compared to where I was,' " she recounted.

The Defense Department official disputed that suggestion, saying, "We have absolutely no indications of any mistreatment."

Anna Kar, Mr. Kar's sister, said she had spent some time with her brother in Tehran, where their mother lives, about six weeks before he traveled into Iraq. She warned him about the dangers of such a trip, she said, but her fears were no match for his determination to complete filming for a documentary he had been trying to make about the Persian ruler Cyrus the Great.

"He had always been a little ashamed of being Iranian," she said, noting that the Iranian revolution and the American hostage crisis in Tehran played out just as her brother finished high school.

"Especially in the Navy, he got a lot of racial slurs," she said. "But reading about Cyrus the Great, he had felt a real sense of pride in what he thought was the real Iran - this tolerant, benevolent empire. And he started on this quest."

After growing up in the United States, Germany and Iran as the child of divorced parents, Mr. Kar drifted for a while after high school, his sister said. He enlisted in the Navy in 1983, partly to earn money for college, and served on the aircraft carrier Ranger, relatives said.

Completing his obligation in 1986, relatives said, he became an American citizen and graduated from San Jose State University in 1990 with a bachelor's degree in marketing. He spent a decade working for technology companies in Silicon Valley and moved to Los Angeles after the technology bubble burst in 2000.

Friends and relatives described him as devoutly eclectic: a liberal who strongly supported the Bush administration's fight against terrorism; a vegetarian and student of the Civil War; a man whose bedroom walls were draped with an American flag and banners memorializing the reggae singer Bob Marley.

In 2002, documents in his files show, Mr. Kar began making contact with archeologists and historians for what was initially to be a pamphlet on the Cyrus the Great, a magnanimous ruler who is reviled as a pagan by some Islamic fundamentalists. Later, with encouragement from Philippe Diaz, a longtime film director and producer who was a friend of his sister's, Mr. Kar decided to make a documentary film.

"He was a first-time director, no question about it," said Mr. Diaz, chairman of the independent film studio Cinema Libre, in an interview. "But he was so determined."

Mr. Diaz said he had agreed to finance the postproduction costs of Mr. Kar's film, which he was shooting on mini-DV format, and was helping him learn to edit. He and others said Mr. Kar had shot perhaps 40 or 50 hours of tape, including interviews with various scholars and footage of archaeological sites in Iran, Afghanistan and Tajikistan - all of which he visited in the fall of 2004.

Filming in Babylon, the ancient city south of Baghdad that Cyrus the Great conquered in 538 B.C., became Mr. Kar's final goal for the project.

"I didn't blame him for wanting to go," said Kamyar Abdi, an anthropologist at Dartmouth College who toured archaeological sites in Iran with Mr. Kar last year. "But I didn't think under the present circumstances that it was a very good idea."