When Donna Lee Peterkin heard the first shots fired, she locked the door behind her and dashed for cover beneath her patient’s bed.

Ms. Peterkin, a registered nurse, comforted the patient, a woman with pneumonia, as shots continued to ring out in the hallway on the 16th floor.

When the commotion subsided, police officers kicked in the door.

“Stay here and don’t come out, don’t come out,” a police officer told them. Ms. Peterkin turned on the television in the room, and reports of wounded colleagues began to flash on the screen.

But Ms. Peterkin could not sit tight.

After the police told her that the gunman, Dr. Henry Bello, had died, Ms. Peterkin slipped out of the room and splashed through the flooded hallways, trying to evade the water raining from the sprinklers, which had been turned on by a fire set by the gunman.

“Peterkin, come,” a doctor screamed. “This patient cannot breathe.”

The patient was turning blue, said Ms. Peterkin, who has worked at the hospital for 25 years. The fire had deactivated oxygen machines, and at least two patients on that floor were left without oxygen.