A staff member quoted by News 4 said that the hospital is under lockdown as police are evacuating the people present inside floor by floor.
Police said the gunman, a former employee identified as Dr. Henry Bello, opened fire inside Bronx Lebanon Hospital in the early afternoon.
Bello, 45, is a family medicine doctor, according to sources.
At least four to six people were injured in the shooting.
The FBI of NY said there was no nexus to terrorism at this time. Their conditions are not yet known. More information will be posted as it becomes available.
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And in Florida last July, a man entered a patient's room and opened fire, killing an elderly female patient and a female hospital worker, police said.
CNN reports that the hospital is "among the largest providers of outpatient services in NY".
The gunman is believed to have barricaded himself on that floor, the Daily News reported, adding that police found a trail of blood leading from the second floor to the eighth.
CBS New York reporter Reena Roy tweeted that the NYPD has confirmed that the shooter is down and no longer a threat.