WASHINGTON, July 1 - Driven in part by fears of terrorism, government secrecy has reached a historic high by several measures, with federal departments classifying documents at the rate of 125 a minute as they create new categories of semi-secrets bearing vague labels like "sensitive security information."

A record 15.6 million documents were classified last year, nearly double the number in 2001, according to the federal Information Security Oversight Office. Meanwhile, the declassification process, which made millions of historical documents available annually in the 1990's, has slowed to a relative crawl, from a high of 204 million pages in 1997 to just 28 million pages last year.

The increasing secrecy - and its rising cost to taxpayers, estimated by the office at $7.2 billion last year - is drawing protests from a growing array of politicians and activists, including Republican members of Congress, leaders of the independent commission that studied the Sept. 11 attacks and even the top federal official who oversees classification.

The acceleration of secrecy began after the 2001 attacks, as officials sought to curtail access to information that might tip off Al Qaeda about America's vulnerabilities. Such worries have not faded; just this week the Department of Health and Human Services sought unsuccessfully to prevent publication of a scientific paper about the threat of a poisoned milk supply on the ground that it was "a road map for terrorists."

But across the political spectrum there is concern that the hoarding of information could backfire. Thomas H. Kean, chairman of the Sept. 11 commission and a former Republican governor of New Jersey, said the failure to prevent the 2001 attacks was rooted not in leaks of sensitive information but in the barriers to sharing information between agencies and with the public.

"You'd just be amazed at the kind of information that's classified - everyday information, things we all know from the newspaper," Mr. Kean said. "We're better off with openness. The best ally we have in protecting ourselves against terrorism is an informed public."

Mr. Kean said he could not legally disclose examples he discovered of unnecessary classification. But others cite cases of what they call secrecy running amok: the Central Intelligence Agency's court fight this year to withhold its budgets from the 1950's and 60's; the Defense Intelligence Agency's deletion of the fact that the Chilean dictator Augusto Pinochet was interested in "fencing, boxing and horseback riding"; and the Justice Department's insistence on blacking out a four-line quotation of a published Supreme Court decision.

Secrecy has long been denounced by liberal watchdog groups like the American Civil Liberties Union. But more conservatives are emerging as skeptics, including Senator John Cornyn, Republican of Texas, whose bill to strengthen the Freedom of Information Act passed the Senate last week. The bill, cosponsored by Senator Patrick J. Leahy, Democrat of Vermont, requires that any legislation creating new exemptions to the Freedom of Information Act explicitly disclose them. It is only part of overhaul efforts proposed by the two.

Mr. Cornyn, a former state attorney general, said he had been trying to persuade his colleagues that freedom of information was not just a concern of the news media. "The people should get the information they need to see if government is doing what they want," he said.

He gets no argument from J. William Leonard, who in his three years as director of the Information Security Oversight Office has waged a lonely battle against overclassification. "I've seen information that was classified that I've also seen published in third-grade textbooks," Mr. Leonard said.

Such missteps may come in part from inexperience. Since 2001, President Bush has extended the power to classify documents to the heads of the Environmental Protection Agency, the Department of Health and Human Services and the Department of Agriculture. At the Agriculture Department, where officials are concerned about agroterrorism, employees can visit the agency's Web site and easily print out a bright-yellow "sensitive security information" cover sheet.

Such labels for unclassified information deemed sensitive have multiplied in recent years, going beyond the traditional "for official use only" to "law enforcement sensitive," "homeland security sensitive" and other vague tags.

"We find there's such a proliferation of these bogus categories," which lack clear rules or definitions, said Lawrence J. Halloran, an aide to Representative Christopher Shays, a Connecticut Republican who held a hearing on excessive secrecy in March.

The secrecy wave has reached obscure outposts of federal power. Wes Addington, a lawyer in Prestonsburg, Ky., who represents mine workers filing safety complaints, said the Mine Safety and Health Administration now denied him documents under the Freedom of Information Act that a few years ago were routinely provided.

"Honestly, I don't understand the reason," Mr. Addington said. "I don't see how releasing this stuff would hurt the government."

A spokeswoman for the safety agency, Suzanne Bohnert, said it began withholding inspectors' notes because their release could "harm ongoing enforcement matters."

Some opponents of secrecy say the Bush administration has created an atmosphere that discourages disclosure. Vice President Dick Cheney won a court battle to keep secret the records of his energy task force. Former Attorney General John Ashcroft issued a memorandum in October 2001 urging officials to be careful to protect sensitive information.

Thomas S. Blanton, director of the National Security Archive at George Washington University, said the administration viewed public access to information as encroaching on executive power.

But a spokesman for the National Security Council, Frederick L. Jones II, said the effect of the Ashcroft memorandum had been "greatly exaggerated" by critics. Mr. Jones blamed the increasing use of e-mail for the rapid rise in classified documents. He added that the president had nominated members for a planned Public Interest Declassification Board that would guard against excessive secrecy.

"The administration is proud of its record of openness," Mr. Jones said.

Mr. Blanton's group, which files hundreds of Freedom of Information requests each year, occasionally gets a glimpse of the arbitrariness of bureaucrats' decisions. In 1999, the Defense Intelligence Agency released its two-page Pinochet biography without deletions. Three years later, it released the same document with half the text deleted, including such comments as "Gen. Pinochet is conservative in his political thinking."

In a recent battle, Steven Aftergood, who directs the Project on Government Secrecy at the Federation of American Scientists, sought C.I.A. budgets for 1947 to 1970. A judge gave him only the 1963 budget, because it had already been released.

"I don't know any intelligence professional who says, 'I'll stake my integrity on the need to protect the 1962 intelligence budget,' " Mr. Aftergood said.

But a C.I.A. spokesman, Paul Gimigliano, said that releasing even old budgets could prove a slippery slope. "The budget remains classified to prevent America's adversaries from piecing together the national security priorities set for the C.I.A.," he said. "This is not secrecy for secrecy's sake."