FBI had interviewed deceased bombing suspect Tamerlan Tsarnaev in 2011, but wondered if it was fair to blame the agency for not anticipating something that happened two years later.

"The ball was dropped in one of two ways," Graham opined. "The FBI missed a lot of things is one potential answer or our laws do not allow the FBI to follow up in a sound, solid way. There was a lot to be learned from this guy. He was on websites talking about killing Americans."

"This was a mistake," he added. "I don't know if our laws are insufficient or the FBI failed, but we're at war with radical Islamists and we need to up our game."

House Intelligence Committee Mike Rogers (R-MI), however, on Sunday insisted that the FBI was not to blame, arguing that the bureau "did their due diligence and did a very thorough job."

Crowley also pressed the Graham on his assertion that surviving Boston bombing suspect Dzokhar Tsarnaev should be treated as an enemy combatant and should be interrogated without being informed of his Miranda rights.

"This man should be designated as a potential enemy combatant and we should be allowed to question him for intelligence gathering purposes to find out about future attacks and terrorist organizations that may exist that he has knowledge of and that evidence cannot be used against him in trial," the senior South Carolina senator argued. "Anytime we question him about his guilt or innocence, he is entitled to his Miranda rights and a lawyer."

"But we have the right under our law -- I've been a military lawyer for 30 years -- to gather intelligence from enemy combatants. And a citizen can be an enemy combatant."