In short, it's unclear whether they were actually involved with any organized jihad groups or just developed a personal obsession with the ideas and copied the methods.
The younger brother was just too much under the influence of the elder, it appears from what I've read. Maybe he looked up to him, and just followed him blindly. Who knows.
As long as we keep figuring out ways to hate each other, we'll keep figuring out ways to kill each other.
Of course, he may decide to confess and plead guilty. There'll be an avalanche of detail either way, books ad nauseam and so on..
Update: wow. WTF is this? :-)
Thanks for all the upvotes...
edit: Carmen Ortiz (prosecutor in Aaron Swartz case and now Boston Bombing/Manhunt) confirms use of public safety exemption