In addition, Boston police have surveillance video of someone bringing multiple backpacks to the blast site, according to CBS News.
Police also confirmed that there was a third explosion, at the John F. Kennedy Presidential Library and Museum. It was not immediately clear how much damage was done or whether it was related to the bombings at the marathon finish line.
The library bombing occurred about 4:30 p.m. and more than a mile from the marathon.
A law enforcement source confirmed to The Post that 12 people were killed and nearly 50 were injured in today's blast.