WASHINGTON — The FBI improperly opened and extended investigations of some U.S. activist groups and put members of an environmental advocacy organization on a terrorist watch list, even though they were planning nonviolent civil disobedience, the Justice Department said Monday.

A report by Inspector General Glenn Fine absolved the FBI of the most serious allegation against it: that agents targeted domestic groups based on their exercise of First Amendment rights. Civil liberties groups and congressional Democrats had suggested that the FBI employed such tactics during the George W. Bush administration, which triggered Fine’s review.

But the report cited what it called other “troubling” FBI practices in its monitoring of domestic groups in the years from after the Sept. 11, 2001, terrorist attacks to 2006. In some cases, Fine said, agents began investigations of people affiliated with activist groups for “factually weak” reasons.

In others, the report said, the FBI extended investigations “without adequate basis” and improperly kept information about activist groups in its files. Among the groups monitored were the Thomas Merton Center, a Pittsburgh peace group; People for the Ethical Treatment of Animals; and Greenpeace USA. Activists affiliated with Greenpeace were improperly put on a terrorist watch list, the report said.

FBI deputy director Timothy Murphy, in a response included with the report, said the FBI was “pleased” that Fine “concludes the FBI did not target any groups for investigation on the basis of their First Amendment activities.” He said the FBI inquiries were based on information about potential criminal activities.

Murphy’s boss, FBI director Robert Mueller III, is cited in the report for unintentionally providing inaccurate congressional testimony about one of the investigations. Relying on information from other FBI officials, Mueller testified in 2006 that the FBI had information that “certain persons of interest” in international terrorism probes were expected to be present at a 2002 antiwar event sponsored by the Merton Center, Fine’s office said.

An FBI agent from the Pittsburgh Field Division attended the rally and was told by his supervisor to look for terrorism suspects, but Fine’s investigators found no evidence that the FBI had information that any terrorism suspects would be there.