Police may need several more days to fully process the crime scene evidence at the site of the Boston Marathon bombing, Boston Police Commissioner Ed Davis said at a press conference this morning.

 It is the "most complex crime scene in the history of our department," he said.

 As of this morning, a cordon was being maintained around a 12-block area and guarded by the National Guard, Davis said. Immediately after the bombing the closure had extended to 15 blocks. Davis said that law enforcement agencies coordinating the investigation planned to gradually reduce the closed area as it was processed, but they will likely need at least two days and possibly more to collect evidence.

 Police and FBI officials again called on members of the public to come forward with any information that could provide a clue to the bombings, particularly photos or video of the area around the time of the explosion.

 Officials also stated that contrary to yesterday’s rumors of more unexploded devices, only two bombs have been discovered, those which exploded during the race. The tally of casualties now stands at 176, including 17 critically injured people and three dead.

 In a separate press conference, Dr. George Velmahos, a trauma surgeon at Massachusetts General Hospital, said his own institution had treated 34 patients in the aftermath of the bomb, including four who required immediate amputations. Two more patients remain at risk of losing a limb, though all 12 of the victims MGH admitted were in stable condition as of this morning.

 "We just completed what the bomb had done," he said of the amputees.

 Velmahos said he believed that the bomb was deliberately packed with shrapnel based on "numerous" metallic items which have been removed from injured people, including uniform "pellets" of some kind; nails; and other sharp objects.