Rep. Dutch Ruppersberger (D-Md.) told ABC News on Sunday that the FBI is investigating "persons of interest" inside the United States connected to the Boston bombing.

"There are persons of interest in the United States," said Ruppersberger, the ranking Democrat on the House Intelligence Committee. "We're looking at phone calls before and after the bombing, this type of investigation."

House Intelligence Committee Chairman Mike Rogers (R-Mich.) added, "There are still persons of interest in the United States that the FBI would like to have conversations with." But "the big unknown," he said, was the six months Tamerlan Tsarnaev spent in Russia.

"I think [Russian authorities] have information that would be incredibly helpful that they haven't provided yet," he said.

On Fox News Sunday, House Homeland Security Committee Chairman Michael McCaul (R-Texas) said that he had discussed the involvement of "trainers" with the FBI.

“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 ... that there was a trainer and the question is where is that trainer or trainers,” he said.

“Are they overseas in the Chechen region or are they in the United States?” he asked. “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.), who sits on the House Intelligence Committee, told CNN's "State of the Union" he also believed the Russians "know more than they're telling us." He said he wanted to know why Russian authorities recorded a conversation between Tamerlan Tsarnaev and his mother, "We don't know that. We haven't received that information from the Russians," he said.

We don't yet know how exactly the brothers became radicalized, he said, though he said there probably is no link to a major terrorist group. “It does look like a lot of radicalization was self-radicalization online, but we don’t know the full answers yet,” he said.