There has been more information released - they have surveillance video from a nearby department store and seem to have a couple of possible suspects seen to have been carrying what appeared to be large/heavy bags. I would imagine as in many cases authorities may not release information publicly that they need to keep confidential until they apprehend those responsible.
Investigators seem to have been reviewing a good deal of photos and video footage, and continuing to scientifically study evidence from the scene. Apparently in the past they've been able to find where materials were manufactured or even sold so it seems as they continue to study clues they'll get closer to finding out how this was carried out.
Condolences to those families who lost loved ones or suffered serious injuries.
edit - And on a photography related note, I saw a reporter (on CNN I think it was) who afterwards said that as it was happening his cameramen was still recording footage as he was running over to the fence to help pull it out of the way for first responders to get through.