CNNBoston mayor Thomas Menino and police commissioner Edward Davis just held a press conference.
Davis said that there is no suspect in custody.
There are reports out of Boston that police are guarding a potential suspect in the hospital.
Davis didn't specifically deny those reports, but said no one is in custody right now.
He also confirmed that there are fatalities, but declined to say how many.
Thomas Menino was present at the press conference, but didn't give any new information.