The FBI improperly investigated some left-leaning U.S. advocacy groups after the Sept. 11, 2001, attacks, the Justice Department said Monday, citing cases in which agents put activists on terrorist watch lists even though they were planning nonviolent civil disobedience.

A report by Inspector General Glenn A. Fine absolved the FBI of the most serious allegation: that domestic groups were targeted purely for their activism against the Iraq war and other political activity, which would have violated their First Amendment rights. Civil liberties groups and congressional Democrats had accused the FBI of employing such tactics during George W. Bush's administration.

But the report cited what it called "troubling" FBI practices in the Bush administration's monitoring of domestic groups between 2001 and 2006. In one instance, the report said, FBI officials falsely said an agent photographed antiwar demonstrators as part of a terrorism investigation, which led FBI Director Robert S. Mueller III to unintentionally give incorrect information about the incident to Congress.

In another, agents investigated members of the environmental advocacy group Greenpeace over their protest activities "with little or no basis," the report said. Agents kept the case open for more than three years, even though no charges were filed, and put the activists on a terrorist watch list, it said.

The groups that were monitored, which also include a Catholic organization that advocates for peace, compared the FBI's actions to questionable domestic spying tactics the bureau usedagainst antiwar demonstrators and others in the 1960s under longtime director J. Edgar Hoover.

"The use of McCarthyite tactics against PETA and other groups that speak out against cruelty to animals and exploitative corporate and government practices is un-American, unconstitutional, and against the interests of a healthy democracy,'' said a statement from People for the Ethical Treatment of Animals, an animal rights group that was among those monitored.

Ken Wainstein, former head of the Justice Department's national security division, said the investigations of the groups reflect the FBI's post-Sept. 11 challenge of transforming into an intelligence organization able to detect and dismantle terrorist plots.

"This isn't McCarthyism or the excesses of the 1960s,'' he said. "This is the Bureau developing the programs to be a fully functioning intelligence agency and trying not to step over the First Amendment lines in the process.''

FBI officials defended their tactics, saying they were trying to protect Americans. They noted that the express purpose of Fine's report was to determine whether agents targeted activists purely for their political beliefs.

"After more than four years of investigation and an exhaustive review of hundreds of investigative decisions the FBI made after the September 11 attacks," said FBI spokesman Michael P. Kortan, the report "did not uncover even a single instance where the FBI targeted any group or any individual based on the exercise of a First Amendment right.''

He added that although Fine had "disagreed with a handful of the FBI's investigative determinations over the course of six years,'' the inspector general "has not recommended any significant modifications to the FBI's authority to investigate criminal conduct or national security threats.''

The FBI's efforts to balance its fight against domestic terrorism with respect for the First Amendment have long been controversial. Under Hoover's COINTELPRO program, halted in 1971, the bureau sought to monitor and disrupt leftist antiwar and civil rights groups by such tactics as infiltrating them with informants.