I wouldn't leap to the conclusion that they have the right people; I would wait for evidence. They seem to have found them with incredible speed - how often has an attack been resolved this quickly? It would not be the first time the wrong person was arrested (and smeared) in a high pressure situation.

They need a head on a pike to save face for the FBI, both in front of the public and for internal credibility in government, law enforcement, and within their own origanization. Imagine how the FBI would look if they couldn't find or apprehend the perpetrators. And the current FBI - the leadership and their superiors, at least - seems much more focused on politics than predecessors.