A secret FBI intelligence unit helped detain a group of war protesters in a downtown Washington parking garage in April 2002 and interrogated some of them on videotape about their political and religious beliefs, newly uncovered documents and interviews show.

For years, law enforcement authorities suggested it never happened. The FBI and D.C. police said they had no records of such an incident. And police told a federal court that no FBI agents were present when officers arrested more than 20 protesters that afternoon for trespassing; police viewed them as suspicious for milling around the parking garage entrance.

But a civil lawsuit, filed by the protesters, recently unearthed D.C. police logs that confirm the FBI's role in the incident. Lawyers for the demonstrators said the logs, which police say they just found, bolster their allegations of civil rights violations.

The probable cause to arrest the protesters as they retrieved food from their parked van? They were wearing black -- a color choice the FBI and police associated with anarchists, according to the police records.

FBI agents dressed in street clothes separated members to question them one by one about protests they attended, whom they had spent time with recently, what political views they espoused and the significance of their tattoos and slogans, according to interviews and court records.

The revelations, combined with protester accounts, provide the first public evidence that Washington-based FBI personnel used their intelligence-gathering powers in the District to collect purely political intelligence. Ultimately, the protesters were not prosecuted because there wasn't sufficient evidence of trespassing, and their arrest records were expunged.

Similar intelligence-gathering operations have been reported in New York, where a local police intelligence unit tried to infiltrate groups planning to protest at the Republican National Convention in 2004, and in Colorado, where records surfaced showing that the FBI collected names and license plates of people protesting timber industry practices at a 2002 industry convention.

Several federal courts have ruled that intelligence agencies can monitor domestic groups only when there is reason to believe the group is engaged in criminal activity. Experts in police conduct say it is hard to imagine how asking questions about a person's political views would be appropriate in a trespassing case.

The Washington case centers on activities that took place April 20, 2002 -- a day of three cacophonic but generally orderly rallies that drew an estimated 75,000 people to the Mall. They included groups demonstrating against the prospect of war in Iraq, numerous supporters of the war, and Palestinians and others rallying for an end to U.S. aid to Israel and for peace in the Middle East.

The police logs for that day show how events developed: Secret Service agents had some concern about a group near the JBG Co. building's garage at 1275 K St. NW just after 5 p.m.

"Intell 53 advises that five members of the anarchist group have entered a parking garage," reads an entry from 5:12 p.m.

Ten minutes later, an entry notes the FBI's role.