I think they are more interested in who his brother was friendly with; what brother was up to when he took that six month "vacation" to Russia, where he learned to make pressure cooker bombs, where he got his other weaponry, who helped him, stuff like that.
They really don't need him to incriminate himself. There's enough on video to incriminate him, and likely more we haven't seen on dash-cam. ''
They probably want to ask him about this:
In yet another twist in the fast-breaking story, New Bedford police said this evening that three people had been taken into custody in their city as part of the bombing investigation.
New Bedford Police Lieutenant Robert Richard said his department assisted federal investigators in executing a search warrant at a home on Carriage Drive in New Bedford, about 10 minutes from the campus of the University of Massachusetts at Dartmouth, where Tsarnaev was a student.
Richard said the FBI took two men and a woman into custody. They appeared to be either fellow college students or fellow residents, he said.
http://www.bostonglobe.com/metro/2013/04/18/mit-police-officer-hit-gunfire-cambridge-police-dispatcher-says/4UeCClOVeLr8PHLvDa99zK/story.html