The FBI is apparently investigating “persons of interest in the United States” who are in some way connected to the Boston Marathon bombing, Rep. Dutch Ruppersberger told ABC News Sunday, highlights the Washington Post. “We’re looking at phone calls before and after the bombing, this type of investigation.” House Intelligence Committee Chairman Mike Rogers echoed the sentiment saying that “we still have persons of interest that we're working to find and identify and have conversations with.” Even if the suspected bombers became extremists out of their own accord, “you still need outside affirmation,” Rogers added.
On Fox News, a Republican lawmaker questioned whether the Boston suspects were trained in how to carry out the attacks and build pressure cooker bombs. "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,” Rep. Michael McCaul, chairman of the House Homeland Security Committee, said.
CBS News' Bob Orr also said the sophistication of the weaponry was one of the key reasons why officials believe others may have been involved. "If they used remote triggers—which seems to be the consensus—it's hard to imagine two guys under the radar could buy all this stuff, build these bombs, carry off the attack and have it all work perfectly," he said. "Where did they practice? And that begs the question did anybody help them train."
Others, however, were quick to emphasize that so far there has been no evidence to suggest the Tsarnaev brothers were part of a bigger plot. “It appears, at this point, based on the evidence, that it's the two of them," Sen. Claire McCaskill said on CBS.