Federal officials familiar with the program maintain that warrants are unneeded for the kind of radiation sampling the operation entails, but some legal scholars disagree. News of the program comes in the wake of revelations last week that, after 9/11, the Bush White House approved electronic surveillance of U.S. targets by the National Security Agency without court orders. These and other developments suggest that the federal government's domestic spying programs since 9/11 have been far broader than previously thought.

The nuclear surveillance program began in early 2002 and has been run by the FBI and the Department of Energy's Nuclear Emergency Support Team (NEST). Two individuals, who declined to be named because the program is highly classified, spoke to U.S. News because of their concerns about the legality of the program. At its peak, they say, the effort involved three vehicles in Washington, D.C., monitoring 120 sites per day, nearly all of them Muslim targets drawn up by the FBI. For some ten months, officials conducted daily monitoring, and they have resumed daily checks during periods of high threat. The program has also operated in at least five other cities when threat levels there have risen: Chicago, Detroit, Las Vegas, New York, and Seattle.

FBI officials expressed concern that discussion of the program would expose sensitive methods used in counterterrorism. Although NEST staffers have demonstrated their techniques on national television as recently as October, U.S. News has omitted details of how the monitoring is conducted. Officials from four different agencies declined to respond on the record about the classified program: the FBI, Energy Department, Justice Department, and National Security Council. "We don't ever comment on deployments," said Bryan Wilkes, a spokesman for DOE's National Nuclear Security Administration, which manages NEST.

In Washington, the sites monitored have included prominent mosques and office buildings in suburban Maryland and Virginia. One source close to the program said that participants "were tasked on a daily and nightly basis," and that FBI and Energy Department officials held regular meetings to update the monitoring list. "The targets were almost all U.S. citizens," says the source. "A lot of us thought it was questionable, but people who complained nearly lost their jobs. We were told it was perfectly legal."

The question of search warrants is controversial, however. To ensure accurate readings, in up to 15 percent of the cases the monitoring needed to take place on private property, sources say, such as on mosque parking lots and private driveways. Government officials familiar with the program insist it is legal; warrants are unneeded for monitoring from public property, they say, as well as from publicly accessible driveways and parking lots. "If a delivery man can access it, so can we," says one.

Georgetown University Professor David Cole, a constitutional law expert, disagrees. Surveillance of public spaces such as mosques or public businesses might well be allowable without a court order, he argues, but not private offices or homes: "They don't need a warrant to drive onto the property -- the issue isn't where they are, but whether they're using a tactic to intrude on privacy. It seems to me that they are, and that they would need a warrant or probable cause."

Cole points to a 2001 Supreme Court decision, U.S. vs. Kyllo, which looked at police use -- without a search warrant -- of thermal imaging technology to search for marijuana-growing lamps in a home. The court, in a ruling written by Justice Antonin Scalia, ruled that authorities did in fact need a warrant -- that the heat sensors violated the Fourth Amendment's clause against unreasonable search and seizure. But officials familiar with the FBI/NEST program say the radiation sensors are different and are only sampling the surrounding air. "This kind of program only detects particles in the air, it's non directional," says one knowledgeable official. "It's not a whole lot different from smelling marijuana."

Officials also reject any notion that the program specifically has targeted Muslims. "We categorically do not target places of worship or entities solely based on ethnicity or religious affiliation," says one. "Our investigations are intelligence driven and based on a criminal predicate."

Among those said to be briefed on the monitoring program were Vice President Richard Cheney; Michael Brown, then-director of the Federal Emergency Management Administration; and Richard Clarke, then a top counterterrorism official at the National Security Council. After 9/11, top officials grew increasingly concerned over the prospect of nuclear terrorism. Just weeks after the World Trade Center attacks, a dubious informant named Dragonfire warned that al Qaeda had smuggled a nuclear device into New York City; NEST teams swept the city and found nothing. But as evidence seized from Afghan camps confirmed al Qaeda's interest in nuclear technology, radiation detectors were temporarily installed along Washington, D.C., highways and the Muslim monitoring program began.

Most staff for the monitoring came from NEST, which draws from nearly 1,000 nuclear scientists and technicians based largely at the country's national laboratories. For 30 years, NEST undercover teams have combed suspected sites looking for radioactive material, using high-tech detection gear fitted onto various aircraft, vehicles, and even backpacks and attaché cases. No dirty bombs or nuclear devices have ever been found - and that includes the post-9/11 program. "There were a lot of false positives, and one or two were alarming," says one source. "But in the end we found nothing."