The FBI in recent years opened investigations into some U.S. activists with little basis, unjustifiably extended the duration of the probes, improperly retained information about activist groups in its files, and classified its investigations of “nonviolent civil disobedience” as investigations into “acts of terrorism,” according to a report released today (PDF) by the Justice Department’s Inspector General.

The FBI activities reviewed by the Justice Department took place from 2001 to 2006, and involved groups including the Thomas Merton Center (a Pittsburgh social justice center), People for the Ethical Treatment of Animals (PETA), Greenpeace, The Catholic Worker (communities of religious pacifists) and a Quaker peace activist.

The report by the Justice Department watchdog didn’t find that the FBI targeted these groups on the basis of their free speech activities — which would be a serious violation of FBI guidelines — but did fault the agency for other reasons, most notably a “factually weak” basis for opening investigations.

“FBI agents and supervisors sometimes provided the [Office of the Inspector General] with speculative, after-the-fact rationalizations for their prior decisions to open investigations that we did not find persuasive,” the report said.

The report also found that that the FBI unnecessarily classified its probes as domestic terrorism investigations, even though some of the potential crimes were trespassing or vandalism — acts not normally considered to be terrorism. This classification resulted in several individuals improperly being placed on terrorism watchlists.

The Inspector General also found that the FBI gave “inaccurate and misleading” explanations to justify its attendance at a 2002 rally against the Iraq war organized by the Merton Center.

The FBI’s director, Robert Mueller, told a Senate committee in 2006 that his agents at the antiwar rally “were not concerned about the political dissent,” but were attempting to identify “persons of interest” expected to attend the rally.

That testimony wasn’t supported by an “extremely troubling” FBI document about the incident, the report noted. The document “described no legitimate purpose for the FBI to attend the event” and “supplied no evidence or even suspicion that any criminal or terrorist element was associated with the Merton Center or likely to be present at the event,” the report said.

The FBI, responding in an appendix to the report, acknowledged the inaccuracies. It said that incorrect information was provided to the FBI director, who then testified inaccurately before Congress.

“The FBI regrets that incorrect information was provided regarding this matter,” Deputy Director Timothy P. Murphy wrote in a letter to Inspector General Glenn Fine.

The surveillance of activists — both on the state level as well as the federal level — has been a recent topic of concern. As we’ve noted, Pennsylvania Gov. Ed Rendell, a Democrat, apologized last week for a state contract with an anti-terrorism consulting firm, which produced a document calling opponents to gas drilling “environmental extremists” and flagged these and other activists as potential threats to the state’s security.

The document’s section about environmental extremism, as we’ve noted, cited an FBI bulletin as the source of the information. In the document, the FBI assessed with “medium confidence” the threat that environmental extremists posed to the energy sector. (FBI “assessments,” under 2008 guidelines from the attorney general, are the agency’s “lowest level of investigative activity,” the Inspector General report said.)

The FBI has not responded to our request for comment.