Federal law enforcement sources say as authorities investigate, they take into account the possibility that it's a terrorist attack -- that includes possibly domestic -- until the Joint Terrorism Task Force can rule that out.
Earlier today, investigators worked to simultaneously look for other devices designed to hurt first responders or others.
They also were working to collect videos from every possible location -- that's an effort that will likely go on for several days. They also are working to identify witnesses.
Some law enforcement believe the key behind finding those responsible is collecting pieces of the detonated devices and matching those up with those law enforcement recovered before they went off. The bomb remnants could have ended up in a variety of places, like rooftops, so the combing could take some time.
"They will look for the pieces," said one law enforcement source who asked not to be named. "Then when they reconstruct the device they will try to match it up to other, known, signatures of bomb makers to see if there is a link to anyone under investigation or other bombs that have been examined. Most bomb makers stick with a single design and use it repeatedly."