One Friday afternoon in August, 2004, a Washington, D.C., attorney named Lynne Bernabei received a package from the Department of the Treasury. The government was investigating one of her clients, the American branch of a Saudi charity called the Al Haramain Islamic Foundation, which had been active in fifty countries. Al Haramain had come under scrutiny, as had many other Islamic charities, after the attacks of September 11, 2001, and Treasury Department investigators believed that Al Haramain’s American branch, which was based in Oregon, had connections to Al Qaeda. In response to a request from Bernabei for evidence against her client, the government had turned over two sets of documents, primarily media reports that referred to other branches of Al Haramain. None of the materials demonstrated a direct connection between the Oregon branch and Al Qaeda.

Bernabei asked for any classified evidence the government might have, arguing that it was impossible to rebut evidence that she couldn’t see. When a third batch of evidence arrived, that August afternoon, the cover letter noted that the enclosed materials were “unclassified,” so Bernabei didn’t give much thought to the last item, a four-page document stamped “Top Secret.” “My impression was that it might have been something that was declassified,” she told me recently.

Bernabei photocopied the materials and forwarded them to the half-dozen clients and attorneys associated with the case. Several weeks later, the Treasury Department concluded its investigation, and declared the Oregon branch of Al Haramain a Specially Designated Global Terrorist entity, citing “direct links” with Osama bin Laden.

Soon afterward, two F.B.I. agents visited Bernabei’s office and informed her that a classified document had accidentally been turned over to her. Bernabei told them that she had received only “unclassified” information, but she agreed to retrieve the document from her files. According to Bernabei, one of the agents suggested that as she looked for the document she should try not to think about what it contained. In the following weeks, F.B.I. agents tracked down the copies that she had distributed. One lawyer for Al Haramain had an electronic copy. The F.B.I. asked to purge it from his computers.

Bernabei said that she and her associates did not appreciate the significance of the document, and the government’s efforts to recover it, until December, 2005, when the New York Times revealed that the Bush Administration had authorized the National Security Agency to employ wiretaps inside the United States without first getting a warrant. The document that the Treasury Department had turned over to Bernabei appears to have been a summary of intercepted telephone conversations between two of Al Haramain’s American lawyers, in Washington, and one of the charity’s officers, in Saudi Arabia. The government had evidently passed along proof of surveillance to the targets of that surveillance, and supplied the Oregon branch of Al Haramain—a suspected terrorist organization—with ammunition to challenge the constitutionality of the warrantless-wiretapping program.

Well before September 11th, U.S. intelligence agencies had suspicions about the connections between Islamic charities and terrorism. Zakat, or charitable tithing, is one of the five pillars of Islam, a duty for observant Muslims, and, by some estimates, Saudi charities raise four billion dollars a year. They establish mosques and community centers, distribute religious literature, and dispatch clerics to spread Wahhabism, the severe strain of fundamentalist Islam that is the official religion of the kingdom. “This is an element of Saudi foreign policy,” Lee Wolosky, a member of the National Security Council in the Clinton and Bush Administrations, told me. “It’s very well coördinated. It happens at the highest levels of the Saudi state.” In 2004, David Aufhauser, who as the Treasury Department’s general counsel oversaw its counterterrorism efforts after September 11th, estimated that in recent decades the kingdom had spent “north of seventy-five billion dollars” on Islamic evangelism.

Al Haramain was established, with help from the Saudi royal family, in 1991. Its headquarters were in Riyadh, with offices in foreign countries. Within a few years, the charity was suspected by the C.I.A. of involvement in terrorism. In 1996, a C.I.A. report suggested that a third of Islamic N.G.O.s “support terrorist groups or employ individuals who are suspected of having terrorist connections”; it named Al Haramain as an example. In 1997, a C.I.A. informant in Nairobi said that the local branch of Al Haramain planned to blow up the U.S. Embassy. According to the Times, a C.I.A. inquiry turned up no evidence of a plot, but after the bombings of the American Embassies in Tanzania and Kenya, the following year, Kenyan authorities ordered Al Haramain from the country. In a trial on the bombings in New York in 2001, prosecutors introduced a collection of business cards that had been seized from the Nairobi home of Wadih el-Hage, an Al Qaeda operative who was eventually convicted for his role. One belonged to Mansour al-Kadi, an Al Haramain official in Riyadh, who was the titular vice-president of the Oregon branch (though he never played an active role there, and no further connection was made between the charity and the bombings).

Aqeel al-Aqil, who was Al Haramain’s director during the nineties, told me by e-mail that he could not control aid and donations once they arrived in areas of conflict, such as Bosnia and Chechnya. “If you give a sack of flour to a needy family,” he said, “you cannot guarantee that some of their mujahideen sons will not eat some of the bread made of that flour.” U.S. authorities, however, believed that the charities must be held accountable. “Historically, Al Qaeda and other terrorist groups have set up or exploited some charities,” Stuart Levey, the Treasury Department’s Under-Secretary for Terrorism and Financial Intelligence, told the Senate Finance Committee earlier this month. “Those who reach for their wallets to fund terrorism must be pursued and punished in the same way as those who reach for a bomb or a gun.”

After September 11th, the F.B.I. assigned Dennis Lormel, a veteran financial investigator, to look into how Al Qaeda secured its funding. “We latched on to charities immediately,” he told me. On September 23, 2001, President Bush signed an executive order authorizing the Treasury Department to “designate” individuals or entities believed to be supporting or “otherwise associated” with terrorism, in order to help shut down what Bush called “the financial foundation of the global terror network.”

Designations amount to a kind of economic embargo: anyone who does business with a designated person risks criminal or civil penalties. The Treasury Department can act more quickly than the police or the F.B.I., who may take action only after an investigation. By preëmptively freezing a suspect’s assets, “the government does not have to watch these dollars continue to flow over a period of months or years as it investigates whether it will pursue criminal charges,” a department spokesman, Andrew DeSouza, told me.

Authorities also need less evidence for a designation than they would for prosecution, and they can rely on evidence that would not be admissible in a criminal trial. Matthew Levitt, who until last year was deputy assistant secretary for intelligence and analysis at the Treasury Department, says that designations involve “an extremely robust process. This is not something that can be done easily or willy-nilly.” But Lormel, who retired from the F.B.I. in 2003, says he would have been “hard pressed” to act on some of the material that Treasury officials used. “Oftentimes, I think they base their evidence on media stories or public-source information, whereas we would never use only that,” he told me.