The FBI gave inaccurate information to Congress and the public when it claimed a possible terrorism link to justify surveilling an anti-war rally in Pittsburgh, the Justice Department's inspector general said Monday in a report on the bureau's scrutiny of domestic activist groups.

Inspector General Glenn Fine said the FBI had no reason to expect that anyone of interest in a terrorism investigation would be present at the 2002 event sponsored by the Thomas Merton Center, a nonviolent anti-war and anti-discrimination group.

The surveillance was "an ill-conceived project on a slow work day," the IG stated in a study of several FBI domestic terrorism probes of people affiliated with organizations such as Greenpeace and the Catholic Worker.

Earlier, in statements to Congress and in a press release, the FBI had described the Pittsburgh rally surveillance by one agent as related to a terrorism investigation.

In a letter to the IG, FBI Deputy Director Timothy Murphy said the FBI regrets that inaccurate information was provided to the FBI director and Congress regarding the basis for the agent's presence at the rally.

Speaking generally of the FBI probes it studied, the IG said a domestic terrorism classification has far-reaching impact because people who are subjects of such investigations are normally placed on watchlists and their travels and interactions with law enforcement may be tracked.

The FBI has broad definitions that enable it to classify matters as domestic terrorism that actually are trespassing or vandalism, the inspector general said.

The IG said the evidence did not indicate that the FBI targeted individuals involved with the groups on the basis of their free-speech activities protected by the Constitution's First Amendment, but rather due to concerns about potential criminal acts.

The IG also concluded that the factual basis for opening some investigations was factually weak and that in several instances there was little indication of any possible federal crime, as opposed to state crimes. In some cases, the IG found that the FBI extended the duration of probes without adequate basis and in a few cases the FBI improperly retained information about the groups in its files, classifying some probes relating to nonviolent civil disobedience under its "Acts of Terrorism" classification.

Regarding the Pittsburgh rally, controversy erupted in 2006 over whether the FBI had spied on protesters at the event several years earlier because of their anti-war views.

At the time, the FBI issued a news release saying the surveillance had been based on an ongoing investigation.

FBI Director Robert Mueller told a Senate hearing that the bureau had been trying to identify a particular individual believed to be in attendance.

The FBI's statements to Congress and the public were not true, said the IG, who found no evidence that the FBI had any information at the time of the event that any terrorism subject would be present.

According to the IG, the Office of the Chief Division Counsel in the FBI Pittsburgh Field Division created a document that said the surveillance was supposedly directed at an individual living in Pittsburgh who was of interest to the FBI based on evidence developed in a terrorism probe.

"We determined this version of events was not true," said the IG.

The inaccurate statements may have been inadvertent, but the IG said it is more likely that the document reflected an effort to state a stronger justification for the surveillance.