Brigham and Women's hospital staff treated injuries at the scene and also received 31 patients, including one who might lose a limb, said Ron Walls, chairman of the hospital's department of emergency medicine.

WSJ: Doctors Treating Boston Marathon Victims Describe Injuries

"These are very high-force, high-impact injuries so they cause a lot of damage to tissue and to bone, " said Ron Walls, chairman of the department of emergency medicine at Brigham and Women's Hospital.

WSJ: Two Explosions at Boston Marathon Finish Line