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This Monday, April 15, 2013 photo provided by Bob Leonard shows bombing suspects Tamerlan Tsarnaev, 26, center right in black hat, and his brother, Dzhokhar A. Tsarnaev, 19, center left in white hat, approximately 10-20 minutes before the blasts that struck the Boston Marathon.
The chairman of the House Homeland Security Committee said Sunday that the FBI is investigating in the United States and overseas to determine whether the suspects in the Boston Marathon bombing received training that helped them carry out the attack.
Dzhokhar Tsarnaev, 19, is charged with joining with his older brother, Tamerlan, who's now dead, in setting off the shrapnel-packed pressure-cooker bombs. The bombs were triggered by a remote detonator of the kind used in remote-control toys, U.S. officials have said.
U.S. officials investigating the bombings have told The Associated Press that so far there is no evidence to date of a wider plot, including training, direction or funding for the attacks.
A criminal complaint outlining federal charges against Dzhokhar Tsarnaev described him as holding a cellphone in his hand minutes before the first explosion.
The brothers are ethnic Chechens from Russia who came to the United States about a decade ago with their parents.
"I think given the level of sophistication of this device, the fact that the pressure cooker is a signature device that goes back to Pakistan, Afghanistan, leads me to believe — and the way they handled these devices and the tradecraft — ... that there was a trainer and the question is where is that trainer or trainers," said Rep. Michael McCaul, R-Texas, on "Fox News Sunday."
"Are they overseas in the Chechen region or are they in the United States?" McCaul said. "In my conversations with the FBI, that's the big question. They've casted a wide net both overseas and in the United States to find out where this person is. But I think the experts all agree that there is someone who did train these two individuals."
Rep. Adam Schiff, D-Calif., a member of the House Intelligence Committee, said he thought it's "probably true" that the attack was not linked to a major group. But, he told CNN's "State of the Union," that there "may have been radicalizing influences" in the U.S. or abroad. "It does look like a lot of radicalization was self-radicalization online, but we don't know the full answers yet."
On ABC's "This Week," moderator George Stephanopoulos raised the question to the chairman of the House Intelligence Committee about FBI suspicions that the brothers had help in getting the bombs together.
"Absolutely, and not only that, but in the self-radicalization process, you still need outside affirmation," responded Rep. Mike Rogers, R-Mich.
"We still have persons of interest that we're working to find and identify and have conversations with," he added.
At this point in the investigation, however, Sen. Claire McCaskill said there was no evidence that the brothers "were part of a larger organization, that they were, in fact, part of some kind of terror cell or any kind of direction."
The Missouri Democrat, who's on the Senate Homeland Security and Governmental Affairs Committee, told CBS' "Face the Nation" that "it appears, at this point, based on the evidence, that it's the two of them."
Homemade bombs built from pressure cookers have been a frequent weapon of militants in Afghanistan, India and Pakistan. Al-Qaida's branch in Yemen once published an online manual on how to make one.
Tamerlan Tsarnaev was an ardent reader of jihadist websites and extremist propaganda, officials have said. He frequently looked at extremist sites, including Inspire magazine, an English-language online publication produced by al-Qaida's Yemen affiliate.
In recent years, two would-be U.S. attackers reported receiving bomb-making training from foreign groups but failed to set off the explosives.
A Nigerian man was given a mandatory life sentence for trying to blow up a packed jetliner on Christmas Day 2009 with a bomb sewn into his underwear. Umar Farouk Abdulmutallab had tried to set off the bomb minutes before the Amsterdam-to-Detroit flight landed.
The device didn't work as planned, but it still produced smoke, flame and panic. He told authorities that he trained in Yemen under the eye of Anwar al-Awlaki, a radical American-born cleric and one of the best-known al-Qaida figures.
A U.S. drone strike in Yemen killed al-Awlaki in 2011.
In 2010, a Pakistani immigrant who tried to detonate a car bomb in New York's Times Square also received a life sentence. Faisal Shazad said the Pakistan Taliban provided him with more than $15,000 and five days of explosives training.
The bomb was made of fireworks fertilizer, propane tanks and gasoline canisters. Explosives experts said the fertilizer wasn't the right grade and the fireworks weren't powerful enough to set off the intended chain reaction.
The father of the two suspects said Sunday that he has postponed a trip from Russia to the U.S. because of poor health.
"I am really sick," Anzor Tsarnaev, 46, told The Associated Press. He said his blood pressure had spiked to dangerous levels.
Tsarnaev said at a news conference Thursday that he planned to leave that day or the next for the U.S. with the hope of seeing his younger son, who is under arrest, and burying his elder son, who was killed. His family, however, indicated later Thursday that the trip could be pushed back because he was not feeling well.
Tsarnaev confirmed on Sunday that he is staying in Chechnya, a province in southern Russia, but did not specify whether he was hospitalized. He is an ethnic Chechen and has relatives in Chechnya, although he and his family spent little time in Chechnya or anywhere else in Russia before moving to the U.S. a decade ago.
He and the suspects' mother, Zubeidat Tsarnaeva, returned to Russia last year and settled in Makhachkala, the capital of neighboring Dagestan, where Tsarnaeva's relatives live.
During the past week, they were both questioned extensively by U.S. investigators who had traveled to Makhachkala from Moscow. They also were besieged by journalists who staked out their home.
Tsarnaev's family said last week that he intended to get to the U.S. by flying from Grozny, the Chechen capital, to Moscow. He and Tsarnaeva left Dagestan on Friday, but their whereabouts were unclear.
Mewanwhle, Boston hospitals say the number of patients being treated for injuries sustained in the marathon bombing continues to drop, two weeks after the attack that killed three and hurt more than 260.
There are currenlty 27 marathon bombing victims in Boston hospitals.