Last Friday the Justice Department acknowledged for the first time that it is using evidence derived from warrantless wiretapping to prosecute someone. That development sets the stage for a Fourth Amendment challenge to a law that gives the National Security Agency amazingly broad discretion to eavesdrop on Americans' phone calls and read their email.

In February the Supreme Court rejected a lawsuit brought by attorneys, journalists, and human rights activists who complained that the NSA's snooping had compromised the privacy of their international communications. The Court deemed that injury too speculative, since the plaintiffs could not prove the government was spying on them.

Not so Jamshid Muhtorov, a refugee from Uzbekistan who was arrested in Chicago last year. The FBI says Muhtorov, a Denver resident who had recently quit his job as a truck driver, was on his way to Syria, where he planned to fight Bashar al-Assad's regime as a member of the Islamic Jihad Union, a terrorist group dedicated to overthrowing the Uzbek government and replacing it with a Muslim theocracy. He was charged with "provision of material support [i.e., himself] to a designated foreign terrorist organization."

Last week the Justice Department revealed that the case against Muhtorov relies on evidence "obtained or derived from acquisition of foreign intelligence information." That means he can challenge the constitutionality of the warrantless wiretapping that Congress authorized when it amended the Foreign Intelligence Surveillance Act (FISA) in 2008.

The FBI says Muhtorov fell under suspicion because he contacted "Muhammad," the administrator of a website linked to the Islamic Jihad Union, which suggests the NSA was monitoring email to the site. Based on Muhtorov's exchanges with Muhammad, it seems, the FBI obtained court orders authorizing it to examine all of Muhtorov's email and tap his phone lines.

The initial information about Muhtorov apparently came from surveillance under Section 702 of the FISA Amendments Act (FAA), which allows the NSA to monitor communications without an individualized warrant as long as collecting foreign intelligence is "a significant purpose" and the "target" is a foreigner believed to be located outside of the United States. That authority is much broader than you might surmise from this particular case.

The American Civil Liberties Union calls the FAA "the most sweeping surveillance law ever passed by Congress." Instead of a specific warrant naming the target, based on probable cause to believe surveillance will reveal evidence of a crime, the law authorizes general warrants based on nothing more than the government's vague assertions and assurances. The government need not identify targets or locations, and it need not even allege that anyone whose communications it plans to monitor is a spy, a terrorist, or any other sort of criminal.

As the ACLU explains, "The absence of an individualized suspicion requirement means that the government can engage in the wholesale collection of Americans' international communications." For example, it can "knowingly and intentionally collect all communications between the New York and London offices of Amnesty International" or "all communications between Human Rights Watch in New York and human rights researchers in South and Central Asia." In fact, "under the FAA the government can obtain all communications between New York and London so long as the ostensible targets for this mass acquisition are non-U.S. persons believed to be in the United Kingdom."

Furthermore, the "target" who is "believed" to be abroad may in fact be in the U.S. When people communicate by email or mobile phones, they can be anywhere in the world. Hence the NSA is effectively authorized to monitor purely domestic communications without a warrant.

During a telephone call recorded by the government, the FBI says, Muhtorov and an associate "cursed whoever might be listening in on their conversations and called upon Allah to punish those who do so." I don't know if law-abiding Americans can count on God to stop the government from eavesdropping on their private communications. But the Supreme Court might.