BOSTON (CBS) – The day after two explosions rocked downtown Boston, killing three and leaving more than 170 injured, Boston hospitals are working to treat the victims.

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Massachusetts General Hospital surgeon Dr. George Velmahos told reporters Tuesday most of the injuries he has seen involve the victims’ lower extremities.

Four patients have had limbs amputated and nails and metal fragments were found in many of the injured, Velmahos said.

“This bomb was obviously placed low on the ground, therefore lower extremity injuries are to be expected,” he said.

Mass. General said their patients range in age from 28 to 71 years old and are Americans.

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Most of the injured remain in the intensive care unit, Velmahos said.

Velmahos said Tuesday afternoon he is confident there will be no additional fatalities at Mass. General.

“It’s an extremely sad day for all of us but even more so for the patients and relatives,” Velmahos said. “We’re saddened by the events, we’re offering emotional support and we’ve pulled all our resources to support all the patients.”

A Boston Medical Center doctor said in a Tuesday afternoon press conference, five of their patients required amputations.

A 5-year-old child remains in critical condition there and several surgeries are planned for the next two days, BMC said.

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Brigham and Women’s Hospital, Beth Israel Deaconess, Tufts Medical and Children’s Hospital are also treating victims from Monday’s explosions.