Pipe bombs were my first guess, a witness claimed to have seen 'suicide bombers' via Twitter, but even he doesn't seem very sure of himself. I'll try to find his post again.

For the time being, some decent updates are coming from this account:

https://twitter.com/911BUFF


And a video of one of the explosions:

http://youtu.be/oeVyJ3UgIfw


Would anybody care to guess whether it was remotely detonated judging by the videos?