While many of the people injured in Monday's double explosion have been treated and released from area hospitals, 17 are still in critical condition.

"Many of them have severe wounds, mostly in the lower part of their bodies," said Dr. George Velmahos, chief of trauma services at Mass General Hospital. "Wounds related to the blast effect of the bomb, as well as small metalic fragments that entered their bodies. Pellets, shrapnel, nails that these bombs had."

Velmahos said that surgeons removed fragments from subcutaneous levels of skin, and deeper parts of the body.

"They are numerous, numerous," he said. "There are people who have 10, 20, 30, 40 of them in their body, or more."

A few of these patients will need surgery again, soon. But Velmahos says he's hopeful that they will all survive.