The incident began at approximately 3 p.m. on the 16th and 17th floors of Bronx Lebanon Hospital, according to multiple publications.
At least three of the shooting victims, including the woman who was killed, are doctors with the hospital. People at the scene treated one of the injured doctors by tying an emergency fire hose into a makeshift tourniquet, according to The New York Times.
Five people were seriously injured and one victim was shot in the leg, police Commissioner James O'Neill said in a press conference outside the building.
Multiple outlets reported that the suspect was Dr. Henry Bello, 45, a former employee at the hospital. He was reportedly seen Friday wearing a white lab coat and armed with a rifle. The weapon, an M16-type rifle, was hidden under his lab coat, according to multiple publications.
Bello is listed as a family physician on the hospital's website.
Speaking outside the hospital Friday afternoon, New York City Mayor Bill de Blasio called the shooting a "horrific situation."
"One doctor is dead and there are several doctors who are fighting for their lives right now," de Blasio said.
Witnesses described the chaos inside the building.
Leonard Oyola, 52, was waiting to see his mother, who was in the intensive care unit at the time of the shooting.
"I saw a doctor come out of the elevator, shaking and full of blood," he told HuffPost. "He was standing next to a doctor who got shot. He told me he was trying to hold his guts inside. He was shaking, he was in shock."
Oyola said he let the doctor use his phone, and was promptly evacuated. He said he still didn't know the status of his 73-year-old mother.
The suspect tried to set himself on fire while inside the building, setting off the sprinkler system, O'Neill said.
Miguel Mercado, 61, still had an IV tube in his arm as he spoke to HuffPost outside the hospital. He had evacuated the building just before undergoing an MRI.
He said the whole thing lasted 15 minutes. He woke up in his hospital bed, "looked out the window and saw all these cops and people running by. I thought, 'God, something happened.'"
Mercado said that as he was leaving the building, he saw water coming out of the elevators.
"Everyone, no matter how bad you were, had to walk down the stairs," he said.
When asked if he'd be let back in, Mercado threw up his hands. "I might just go home," he said. "But my shoes and my backpack are still up there."
Jesselyn Cook contributed reporting.