For years, government analysts and watchdog groups have been warning about a specific kind of terrorist threat: the “lone wolf” white supremacist.

But they have been hampered by the growing decentralization of a movement that increasingly organizes online. They have had to broaden their mission to address the pressing concerns over international terrorism. And they have butted up against a persistent problem — figuring out who among the throngs that espouse their constitutionally protected hate speech will one day take it upon themselves to commit an atrocity.

The difficulties came into focus Sunday when Wade Michael Page, a 40-year-old Army veteran with ties to “white power” groups, opened fire in a Wisconsin Sikh temple. The FBI and watchdog organizations had known of Page, according to reports, but had not viewed him as a threat before the massacre, which took six lives and ended with Page’s death. (Despite earlier reports that police had gunned down Page during the rampage, the FBI disclosed Wednesday that he had shot himself in the head after being struck by police gunfire.)

Authorities are still investigating the shooting at the temple and have not released a motive. But the episode has reignited questions about the nation’s monitoring of domestic terrorist threats and raised fears of more attacks.

“Over the last 10 years, the United States has faced a major threat from jihadists linked with al-Qaeda,” said Evan Kohlmann, a terrorism analyst who tracks extremist Web sites. “But you can’t leave all your defenses down to other threats, and there’s no doubt been an explosion in growth in right-wing hate groups in the last four or five years.”

According to the Southern Poverty Law Center, the number of hate groups has risen by 65 percent since 2000. There were 148 “patriot groups,” such as the one that gave rise to Oklahoma City bomber Timothy Mc­Veigh, at the end of 2008, according to the organization’s count. That number, which includes armed militias, had risen to 1,274 in 2011.

Analysts say the increase is partly a response to the economic downturn, which has fed conspiracy theories about the potential collapse of the U.S. monetary system, as well as the election of the nation’s first African American president.

Officials with the Department of Homeland Security and the FBI say they have a robust program aimed at rooting out homegrown threats. Last month in Florida, more than a dozen members of the neo-Nazi group American Front were charged in connection with an alleged plot to fight a “race war.” The charges grew out of a two-year FBI domestic terrorism investigation, according to local news reports.

But experts say law enforcement has struggled with a number of challenges, both practical and political.

Investigators are hindered by laws that protect civil liberties but prevent them from tracking people engaged in hateful speech unless there is a possible or suspected crime.

“No matter how offensive to some, we are keenly aware that expressing views by itself is not a crime and the protections afforded under the Constitution cannot be compromised,” said Paul Bresson, an FBI spokesman.

The rise in right-wing extremism has coincided with a technological shift that allows people to plug into hate groups anonymously online, rather than show up at meetings or rallies. That gives cover to potential “lone wolf” terrorists, who can become radicalized alone in front of their computers.

Such individual actors can be very difficult to thwart, even with the best intelligence, said J.M. Berger, a terrorism analyst.

“There is definitely an unknowable aspect,” he said. “The FBI has profilers who are trained in psychology and look for potential violent actions. Even for profilers, you’re never going to be at 100 percent.”

Efforts to target “right-wing extremists” have also run into political opposition. In 2009, the Department of Homeland Security issued a nine-page report warning of the rise of anti-government militias, anti-tax radicals such as the “sovereign citizen” movement, and white supremacist groups.

Singled out in the report were “lone wolves and small terrorist cells.”

But the report was assailed by conservatives. In part, critics objected to a suggestion that some disgruntled service members could become embroiled in racist movements upon returning to civilian life. The report was issued around the time the tea party movement was gathering steam and defending itself against allegations of racism.

The administration distanced itself from the findings. Daryl Johnson, the senior analyst who penned the report, said the department then disbanded the domestic terrorism unit and reassigned its members to international threats.

Johnson, who now works as an independent consultant, said he believes the Obama administration made a calculation: International terrorism would be a higher priority than domestic terrorism because of the potential for mass casualties and the resulting national trauma.

The reality, Johnson said, is that the toll of domestic terrorism can be severe.

“Each individual incident might not have a high body count, but if you track it over time, it starts to add up, and in the communities that are affected by these attacks, it causes long-term effects,” he said.

Homeland Security officials, however, say that the department has been heavily involved in training local law enforcement to deal with domestic threats such as violent white supremacist groups and that it remains a high priority.

“The Department of Homeland Security protects our country from all threats, whether foreign or homegrown, and we know that violent extremism is neither constrained by international borders, nor limited to any single ideology,” spokesman Matt Chandler said in an e-mail.