Shots rang out at about 2:55 p.m. on the 16th floor at Bronx-Lebonon Hospital, located in the Bronx.
Police said a former employee at that hospital -- identified as 45-year-old Dr. Henry Bello -- opened fire with an assault rifle, injuring six people on the 16th floor.
PHOTOS: Images from the scene
Law enforcement sources said Bello went to the 16th floor asking for a specific doctor, apparently to settle a longstanding grudge.
Told that doctor wasn't there, Bello responded by opening fire with the AR-15 hidden under his lab coat.
Bello then went to the 17th floor, where he encountered a female doctor in a common hallway and fatally shot her, apparently at random. Bello attempted to set himself on fire, and shot himself in the head.
The hospital's sprinklers were activated during this time, knocking down the fire.
Police found Bello lying on the floor with the assault rifle nearby. He was also wearing a white medical-type coat at the time.
New York City Mayor Bill de Blasio said in addition to the doctor who was killed, some of those wounded are also doctors who "are fighting for their lives."
Watch the mayor's full remarks here:
Five of the victims are listed in critical condition. One is in stable condition.
The victims largely suffered gunshot wounds to the head, chest and abdomen, a surgeon said. The most seriously wounded was shot in the liver, said the surgeon, who spoke on the condition of anonymity because he was not permitted to speak publicly.
The mayor added that this appears to be a workplace dispute and not an act of terror.
RELATED: Witnesses recount horror inside deadly Bronx hospital shooting
People inside the hospital took cover, blocking off door access and taking cover. Someone inside the hospital posted this photo showing a barricaded door:
People were seen leaving the hospital with their hands up, being led out by police.
The FBI confirmed this shooting does not appear to be terror-related:
NYPD is handling Bronx shooting. At this time there is no nexus to terrorism; we are monitoring situation. @NYPDnews— FBI New York (@NewYorkFBI) June 30, 2017
The ATF said it was sending agents to assist with the situation:
BREAKING: ATF New York special agents are responding to assist @NYPDnews partners at the Bronx-Lebanon Hospital shooting incident.— ATF HQ (@ATFHQ) June 30, 2017
Bello resigned from Bronx Lebanon Hospital two years ago before he was terminated amid sexual harassment allegations, police sources told ABC News.
(The Associated Press contributed to this report.)