(CBS News) -- As the Boston Marathon bombing investigation continues, investigators are delving deeply into the lives of Tamerlan and his younger brother Dzhokhar Tsarnaev for any clues as to what set them on a path toward terrorism. Authorities still believe the two acted alone, but they continue to probe potential outside influences.
Key lawmakers are questioning whether the suspected bombers got help. Rep. Michael McCaul, R-Texas, said in a TV appearance on Sunday, "I think the experts all agree that there is someone who did train these two individuals."
Up until now, authorities have said there's no evidence suggesting the brothers were part of a larger terrorist group. Still, House Intelligence Committee chairman, Rep. Mike Rogers, R-Mich., admitted the FBI wants to interview anyone who may have known about the brother's alleged plot. Rogers said, "We still have persons of interest that we're working to find and identify and have conversations with."
The Tsarnaevs' mother may hold the key to some answers. In 2011, Russian authorities secretly recorded a phone call between her and Tamerlan Tsarnaev in which the two vaguely discussed "jihad." The FBI says it only learned the details of that conversation days ago, even though Russian authorities asked them to look into Tamerlan in 2011.
Sunday on "Face the Nation," Sen. Lindsey Graham, R-S.C., said, "The warning letters the FBI received in March of 2011 and the CIA, I think in November 2011, included the mother as someone to be worried about."
CBS News senior correspondent John Miller, a former assistant FBI director, said on "CBS This Morning," "This is one of those cases where we're looking for the radicalizer, we're looking for the bomb maker, and as the case unfolds -- and I always say this can change -- we're coming into our third weekend of this. But they've got a long way to go. So far, it looks like these were the actions of Tamerlan Tsarnaev, maybe egged on by his mother, but there doesn't seem to be a giant organization behind it."
The suspects' mother, Zubeidat Tsarnaeva, has repeatedly denied that she or her sons had any involvement with terrorist organizations.
As for the suggestion by some in Congress that the Tsarnaev brothers had some sort of training, Miller said, "It suggests what members of Congress know about bomb making. If you look at the ... instruction page that gave them the layout for how to make the pressure cooker bomb, they're saying the remote control aspect is something that they would have had to go somewhere else, well, scroll down that page in the magazine and the very next picture after the pressure cooker is how to do the remote control. This can be done, and it can be done by dummies."
For now, both parents have put off plans to travel to the U.S. The Tsarnaevs' father was hoping to visit his son and claim the body of his older one, but he says he's too ill to travel. The mother has said she is afraid if she steps foot in the U.S., she'll be arrested.
Investigators are also talking to a mystery man named "Misha." Zubeidat Tsarnaeva says she and Tamerlan Tsarnaev turned more deeply to the Islamic faith because of Misha's influence. Miller said of Misha, "He's been cooperative, according to people I've spoken to. They asked him for all the stuff they ask for in these things. 'Can we have your computer? Can we go over your phone?' They're going to want to mirror his hard-drive, see what his contacts were. And they see he handed everything over, explained the relationship, said he wasn't a radicalizer. He is a guy who is now from Rhode Island who was up in Boston at the time, knew the family, but he doesn't emerge as a fiery preacher or anybody of significance. So it's interesting to hear the family version of this (person) who, to use their words, took over their son's brain. And this guy who said, 'I gave him some perspectives on life, but that was about it.'"
Meanwhile, the Tsarnaev's 19-year-old son and the lone surviving suspect -- Dzhokar -- remains at a federal medical detention center 40 miles from Boston. He's locked inside a 10-by-10-foot cell with a steel door and a slot for food.