Federal investigators suspect that Tamerlan Tsarnaev, left, and Dzhokhar… (Lowell Sun and Robin Young )
WASHINGTON -- Congressional leaders briefed on the FBI’s investigation of the Boston Marathon bombing said Sunday it was still possible that the two Tsarnaev brothers received help and support from others in building the bombs and carrying out the lethal attack.
The lawmakers said the FBI is still seeking to interview “persons of interest” in the United States who may know something about the bombings, or about the brothers' actions, that could help in the investigation.
Since the capture of 19-year-old Dzhokhar Tsarnaev on April 19 and the death of his 26-year-old brother, Tamerlan, U.S. officials have said the siblings appeared to have acted alone and were not part of a wider conspiracy.
But Rep. Michael McCaul (R-Texas), chairman of the House Homeland Security Committee, said it was too early to rule out the possibility that others were involved. He criticized what he called a “rush to judgment” by some in the Obama administration.
"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 … and the way they handled these devices and the tradecraft leads me to believe there was a trainer,” McCaul said on "Fox News Sunday."
"Are they overseas in the Chechen region or are they in the United States?" he said. "In my conversations with the FBI, that's the big question. They've cast 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."
Two members of the House Intelligence Committee said investigators want to speak with several people in the U.S. and others abroad.
“There are still persons of interest in the United States that the FBI would like to have conversations with,” said Rep. Mike Rogers (R-Mich.), chairman of the House intelligence panel.
“We’re looking at phone calls before and after the bombing,” added Rep. C.A. Dutch Ruppersberger (D-Md.). Both appeared on ABC’s "This Week."
Sen. Claire McCaskill, a Missouri Democrat who is on the Senate Homeland Security Committee, said investigators have to yet to find evidence pointing to a larger plot.
“There is no evidence at this point that these two were part of a larger organization, that they were in fact part of a -- some kind of terror cell or any kind of direction,” McCaskill said on CBS’s “Face the Nation." “It appears, at this point, based on the evidence, that it's the two of them.”
On Saturday, a U.S. counter-terrorism official told the Los Angeles Times that Russian authorities had secretly wiretapped a 2011 conversation between a man believed to be Tamerlan Tsarnaev and his mother discussing the idea of jihad.
The Russian Federal Security Service, or FSB, also intercepted a second telephone call between the mother, Zubeidat Tsarnaeva, and another man living in southern Russia who has been the subject of a separate FBI investigation, the official said.
The vague substance of the calls was shared only a few days ago with U.S. officials investigating the April 15 bombing near the finish line of the Boston Marathon, which killed three people and injured more than 260.