Federal officials say the bombs used to attack the Boston Marathon were detonated by remote control. But it's not yet clear what the detonation device was. The charges against Dzhokhar Tsarnaev state he was using a cellphone moments before the blasts. On the campus of the Massachusetts Institute of Technology today, about 4,000 people attended a memorial service for campus police officer Sean Collier. He was killed in what officials say was an ambush by the bombing suspects.