* FBI had questioned one of the suspects in 2011
* U.S. lawmakers wonder why agency didn't take more action
* FBI says it didn't find any "terrorism activity" (Adds Tsarnaev's citizenship bid was scrutinized)
By Susan Cornwell and Mark Hosenball
WASHINGTON, April 21 (Reuters) - U.S. lawmakers asked on Sunday why the FBI had failed to spot the danger from one of the Boston Marathon bombing suspects, and they complained it was one of a series of cases in which someone the agency had investigated had later taken part in attacks.
House of Representatives Homeland Security Committee Chairman Michael McCaul wrote to the FBI and other officials asking why Tamerlan Tsarnaev did not raise suspicions after Russia asked the bureau to investigate him two years ago.
"Because if he was on the radar and they let him go, he's on the Russians' radar, why wasn't a flag put on him, some sort of customs flag?," McCaul, a Texas Republican, said on CNN's "State of the Union" program. "And I'd like to know what intelligence Russia has on him as well."
The FBI interviewed Tsarnaev, the elder of two ethnic Chechen brothers suspected in the Boston bombing, in 2011 shortly after Russia's Federal Security Service asked the agency to look into him as a possible Islamist radical who might soon travel to Russia.
Asked on Sunday about lawmakers' concerns, the FBI said it had no further comment beyond a statement it issued on Friday night when it said it "did not find any terrorism activity, domestic or foreign" after speaking to Tsarnaev and checking his travel records and Internet activity.
Less than a year after the FBI interview, Tsarnaev did in fact travel to the volatile Dagestan region of southern Russia on a six-month trip out of the United States. Much of what Tsarnaev did on that trip is still a mystery to U.S. investigators.
Neighbors contacted by Reuters say Tsarnaev spent at least a few weeks in Dagestan, a predominantly Muslim region in the North Caucasus mountains where Islamist militants have long been a thorn in the side of governments in Moscow.
Republican Representative Peter King of New York told "Fox News Sunday" he wondered why the FBI did not take more action after Tsarnaev returned to the United States last year and put statements on his website "talking about radical imams."
Tamerlan Tsarnaev was not put on any no-fly list of suspected terrorists, U.S. officials said. But his brush with the FBI did raise concerns when he applied for U.S. citizenship last year, a source close to the bombing investigation said.
Officials of the Homeland Security Department decided to give his application extra scrutiny because of the FBI interview and also due to an allegation against him of domestic abuse on a girlfriend in 2009, the source said. The citizenship application was still under consideration when Monday's bombing happened.
'DROPPED THE BALL'
Tamerlan Tsarnaev was killed in a shootout with U.S. police. and his brother Dzhokhar, 19, remained hospitalized in serious condition on Sunday, unable to speak. Three people were killed in Monday's bombing and 176 were injured.
Republican Senator Lindsey Graham of South Carolina said "the FBI or the system dropped the ball" on the elder Tsarnaev. Graham told CNN that U.S. laws do not allow the FBI to follow up enough even if it does spot danger.
Democratic Senator Charles Schumer of New York told CNN "there's certainly a lot of questions" about the FBI's handling of the case.
One U.S. counterterrorism official urged perspective. "If we thoroughly investigated every one of these terrorism tips we get, we'd never get anything done," he said.
A senior U.S. law enforcement source said that the number of tips received from Russian intelligence to the FBI each year is "not that many."
But nationally, he said, the FBI receives at least 100 terrorism tips a day - from the public, local and state law enforcement, other federal agencies and the intelligence community.
House Intelligence Committee Chairman Mike Rogers, a former FBI agent, defended the agency. The Michigan Republican said the FBI had performed a "very thorough" review of the older brother in 2011, but then it failed to receive further cooperation from Russia.
"That case was closed prior to his travel, so I don't think we missed anything," Rogers said on NBC's "Meet the Press."
"At some point they (the FBI) asked, is there more clarifying information, and never received that clarifying information, and at some point they have nothing. You can't ask them to do something with nothing," Rogers said.
But McCaul and King said the handling of Tamerlan Tsarnaev's case looked like it was part of a pattern.
The 26-year-old "appears to be the fifth person since Sept. 11, 2001, to participate in terror attacks despite being under investigation by the FBI," the pair said in a joint letter.
They named the others as Anwar al-Awlaki, a U.S.-born cleric and leader of al Qaeda's affiliate in Yemen who was killed in a U.S. drone strike; David Headley, an American who admitted scouting targets for a 2008 Islamic militant raid on Mumbai; Carlos Bledsoe, who killed an Army private outside a military recruiting office in Arkansas in 2009; and Nidal Hasan, who is accused of killing 13 people at Fort Hood, Texas, in 2009.
In addition, Umar Farouk Abdulmutallab, who tried to bring down a U.S. jetliner over Detroit on Christmas Day 2009, had been identified to the CIA as a potential terrorist, the letter said, adding the cases "raise the most serious questions about the efficacy of federal counterterrorism efforts."
The McCaul-King letter asked for all information the U.S. government had on Tamerlan Tsarnaev before April 15. It was also addressed to Director of National Intelligence James Clapper and Homeland Security Secretary Janet Napolitano. (Additional reporting by John Shiffman, Aruna Viswanatha and Mark Hosenball; Editing by Alistair Bell and Eric Beech)