WASHINGTON — Long before he pulled the trigger, private groups were quietly monitoring the man who killed six worshippers at a Sikh temple in Wisconsin. The FBI was not.

Constrained by the First Amendment and stung by past abuses, federal officials are supposed to limit their oversight of extremists to those actively promoting force or violence. And until the moment he walked into the Sikh temple on Sunday in suburban Milwaukee, a bureau spokesman said Tuesday, Wade Michael Page had not been on the radar of the FBI.

“Law enforcement is challenged every day to balance the civil liberties of U.S. citizens against the need to investigate activities of possible criminal conduct,” FBI spokesman Paul Bresson said. “No matter how offensive to some, we are keenly aware that expressing views by itself is not a crime.”

Private groups can fill the gap, as several did by keeping tabs on Page. But for private and public officials alike, hate monitoring also can pose vexing problems.

“When we know that an extremist is talking about taking violent action, we will definitely share that information with the Justice Department,” Marilyn Mayo, co-director of the Anti-Defamation League’s Center on Extremism, said Tuesday, “but there was no reason to believe he was going to take this kind of action.”

In the wake of the Sikh temple shooting, the Anti-Defamation League, or ADL, and the Southern Poverty Law Center both revealed that Page had previously come to their attention; reportedly, as early as 2000. The ADL’s researchers, for instance, had analyzed pictures of Page playing guitar, adorned with tattoos associated with a self-described white power group called the Hammerskins.

“We have been monitoring him for the past few years,” Mayo said. “Certain people will stand out for their activities.”

The private files on Page, in turn, were part of the two groups’ broader monitoring efforts, which are extensive.

The Southern Poverty Law Center, for one, has identified what it calls “1,018 known hate groups” nationwide, including 84 in California, 55 in Florida, 45 in Texas and 34 in North Carolina. The ADL publishes an online “Extremism in America” encyclopedia, whose entries range from the Nazi Low Riders, founded in California, to the Kansas-based Westboro Baptist Church, whose members won a major Supreme Court free-speech case last year concerning protests outside military funerals.

No constitutional limits impede such private monitoring of groups and individuals, though the organizations still must be careful in their characterizations.

Several years ago, for instance, retired University of Massachusetts political scientist Guenter Lewy successfully sued the Southern Poverty Law Center after the center reported that his skeptical views on what many call the Armenian genocide were shaped by financial ties to Turkey. Under legal pressure, the center apologized and paid Lewy an undisclosed sum.

Last month, court records show, a Texas-based federal judge dismissed a libel suit filed by a former Arizona sheriff named Richard Mack against the same organization. Although a Southern Poverty Law Center publication had erroneously reported that Mack had urged listeners to “take out some” IRS agents, under a story headlined “Willing to Kill,” the judge determined Mack had filed his lawsuit five weeks too late.

A Southern Poverty Law Center spokesman could not be reached Tuesday.

Federal officials, too, must take care even as they keep their eyes peeled.

In early April 2009, the Department of Homeland Security’s Office of Intelligence and Analysis issued a report warning about “right-wing extremism,” including the possibility that “disgruntled military veterans” would be recruited by hate groups. Within a week, under fire by veterans groups, the department retracted the report.

“We are on the lookout for criminal and terrorist activity, but we do not, nor will we ever, monitor ideology or political beliefs,” Homeland Security Secretary Janet Napolitano said at the time.

Page was an Army veteran, administratively discharged; reportedly, after being drunk on duty.

Napolitano’s insistence that her department’s 240,000 employees don’t monitor ideology or political beliefs followed revelations that past administrations had done just that. If government investigators sometimes seem gun-shy now, even as they conduct some monitoring, they are reacting in part to past misfires.

The FBI, to take one of many examples, carefully watched University of California-Berkeley free-speech activist Mario Savo starting in the 1960s. The files, now publicly available online, reveal how such government monitoring can be both mundane and unsettling.

“It was ascertained by means of a suitable pretext by a special agent of the FBI . that Mario Savo presently resides at 19 1/2 25th Ave., Venice and that this individual is unemployed,” one FBI memo noted on Oct. 29, 1971.

But the political revulsion over domestic spying also softened following the 9/11 terrorist attacks, which prompted Congress to empower investigators and prompted the investigators to become more aggressive.

The FBI’s latest “Domestic Investigations and Operations Guide,” made available last year, specifies that while no investigations will take place “based solely on the exercise of First Amendment activities,” this protection is limited.

“Even heated rhetoric or offensive provocation that could conceivably lead to a violent response in the future is usually protected,” the guidebook states, but “the law does not preclude FBI employees from observing and collecting any of the forms of protected speech and considering its content.”