Dzokhar Tsarnaev, the surviving Boston bombing suspect, is is answering investigators in writing from his hospital bed.
Some lawmakers say he should be questioned as an enemy combatant, but tried in a federal court.
"He's going to be convicted. I want the intelligence so we can save American lives, and that can only be done I believe, effectively, if he's treated as an enemy combatant and does not have access to a lawyer," New York Congressman Peter King said on Fox News Sunday.
"They don't need his confession to get him into trial. So I don't think we have to cross the line," New York Senator Chuck Schumer countered on CNN State of the Union.
There are lots of questions about why the FBI didn't do more in 2011 after inquiries about the older Tsarnaev's ties to radical groups.
"I want to know how the FBI or the system dropped the ball when he was identified as a potential terrorist," said South Carolina Senator Lindsey Graham. ""This was a mistake. 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."
Tamerlan Tsarnaev spent six months in Russia last year.
The FBI interviewed him before he left.
"We want to know why after he was interviewed, and after a foreign intelligence service raises a flag on the individual, why he was allowed to travel abroad," says Texas' Rep. Mike McCaul.
Some lawmakers insist the FBI did as much as it could.
"At some point they have nothing, and so you can't ask them to do something with nothing," says Michigan's Rep. Mike Rogers.
Meanwhile, the FBI is disputing the suspects' mother's claim that the older brother was interviewed in Boston after the bombing.