Trauma doctors in Boston continued the grisly task Tuesday of removing limbs from those gravely wounded after dual bombs were detonated at the Boston Marathon.

Surgeons and trauma doctors have been working round the clock since the injured began flooding emergency rooms Monday afternoon. Injuries included burns, shrapnel wounds and ruptured ear drums after two bombs went off at the finish line, four hours into the annual racing event.

The bombings killed three and injured 176. Seventeen of the injured are in critical condition.

Massachusetts General Hospital's chief of trauma surgery, Dr. George Velmahos, said Tuesday his hospital treated 31 victims and has performed four amputations. At least two more patients have limbs that could still require amputation, he said.

Velmahos said most of the amputations were above the knee, since nearly all injuries involved patients’ lower extremities. In many cases, there was no hope of saving the limbs.

“We just completed the ugly job that the bomb did,” he said. “These patients came to us with completely mangled, destroyed extremities, hanging by shreds of muscle and skin. We had to go through the unfortunate task of completing the amputation.”

There were reports that two young brothers, who were waiting at the finish line, both lost a leg in the blast.

The bombs detonated were made out of pressure cookers and hidden inside duffel bags.

Velmahos said doctors removed "a variety of sharp objects," including pellets and nails, from the wounds of victims -- items which would be consistent with the AP report about metal objects crammed inside pressure cookers.

The objects appeared to be shrapnel, meaning they came from the bomb itself and not the environment.

A number of people also had to be treated for ruptured eardrums after the blasts, according to Velmahos.

He said despite the trauma they’ve suffered, some patients who underwent amputation have told him they are grateful it wasn’t worse.

“They are really amazing people, some of them woke up today with no leg, and they told me that they’ve happy to be alive.”

He said an “amazing” number of people turned up at the hospital to donate blood since Monday.

“I don’t think there is a single person in Boston that hasn’t suffered an emotional toll,” Velmahos said. We’re all extremely moved and shattered by the event.”

However, doctors and trauma surgeons have been trained to “set our emotions aside” and take care of the patients, he added.

He noted several members of his team have experience treating the wounded overseas, including in war zones.

Boston Medical Center Trauma Surgeon Dr. Tracey Dechert said Tuesday her hospital had conducted amputations on five patients, four of whom were spectators at the marathon.

Among those being treated for trauma injuries is a 5-year-old boy, who is in critical condition.

Operations have been scheduled for several trauma patients over the next few days.

Patients appeared “remarkably calm” despite all they’ve been through, Dechert said.

“Most of them have family and loved ones with them, which is nice,” she said. “They seem to be handling this very well, given the circumstances.”