America’s top intelligence official said Thursday that there is no evidence so far that the Boston Marathon bombers had help from foreign terrorist networks.

“At this point, I haven’t seen anything that raises a concern there was a bigger plot, but we’re still investigating,” James Clapper, the director of national intelligence, said in an interview. He was responding to a query about a recent newspaper story citing two Russian militants as possible accomplices.

Clapper’s comments are another sign that the bombings, allegedly by Tamerlan and Dzhokhar Tsarnaev, were probably a homegrown operation, perhaps closer to the mass shootings in Newtown, Conn., than to an al-Qaeda plot.

“They may have acted alone, but we need to wait until the investigation . . . [is] completed before we draw that conclusion,” Clapper said. “It gets into how people radicalize. . . . It has to do with personal grievances, and how much they are fed by jihadist Web pages.”

With such domestic plots, Clapper said, there are sharp limits on what the 17 intelligence agencies that report to him can do to collect information. The younger brother, Dzhokhar, was a U.S. citizen, and Tamerlan’s residential status made him what’s known as a “U.S. person,” giving both of them protection from surveillance by the FBI and other intelligence agencies.

“There are restrictions on how intrusive we can be in monitoring U.S. citizens,” Clapper said. He cautioned that more aggressive efforts to connect the dots could create serious civil liberties issues, and he asked pointedly: “Does the public want us to be more intrusive in monitoring their Internet activity, listening in to their cellphone calls, monitoring their travel overseas? Do you want us to do this to you?”

Critics have argued that the FBI and other agencies botched the investigation of the Tsarnaevs. They note that the CIA was alerted by Russian intelligence in 2011to concerns about Tamerlan, prompting an FBI interview of him that year. He was still able to travel to Russia in January 2012 and back to the United States that July without further queries.

But Clapper said the intelligence community was properly following rules for entering and using names on the Terrorist Identities Datamart Environment (TIDE) database that’s maintained by the National Counterterrorism Center (NCTC). He said that the FBI interview of Tamerlan didn’t produce enough corroborating evidence of terrorist links to prompt his inclusion on a no-fly or special screening list and that, under TIDE’s rules, the name was dropped from the roster.

“We had nothing on him. All we had was a Russian query. We asked three times for clarification. The FBI did a very thorough check, asked back for clarification, but never got any,” Clapper said.

Through the interview, Clapper raised his voice several times to say that his job is not to oversee perfect surveillance but rather to manage intelligence under privacy rules that properly limit the government’s ability to collect or keep information. Not long before the Tsarnaev case broke, he noted, his office’s inspector general had conducted an investigation of the NCTC compliance with the rules for maintaining the TIDE database. “They got a very good report card on that,” Clapper said.

Given the limits on government surveillance of homegrown extremists, what’s the strategy for preventing domestic terrorism? Basically, it focuses on outreach to Muslims and other communities to get their help in monitoring and disrupting mobilization for terrorist activities.

Mere radicalization ­— visiting jihadist Web sites or talking about extremist groups — “is 100 percent legal,” one analyst said. The crossover comes when a person is mobilized for action, but such conversion is hard to detect because it usually takes place within the “personal zone,” as was apparently the case with the Tsarnaevs. Even when they visited jihadist Web sites or posted inflammatory messages, the Tsarnaevs’ activities couldn’t be monitored legally.

Analysts say talking to Muslim communities — through community awareness briefings at mosques, community centers and other meeting places — is the most effective tool for preventing homegrown Islamist extremism. If Muslim families feel part of a larger American community, they have a greater stake in monitoring and preventing violence. That’s not Dr. Phil talking, but some of the nation’s hard-nosed counterterrorism specialists.

Clapper and his analysts have concluded that the right answer to homegrown plots isn’t police-state surveillance but good community policing. It won’t stop the occasional plot by people who get radicalized, like the Tsarnaevs, but as one analyst noted, “it’s not likely to spike into a mass phenomenon.”

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