NEW YORK — It took two years, but Dr. Henry Bello made good on his threats.

After he was forced to resign as a family medicine doctor amid sexual harassment allegations, he told colleagues he would be back to kill them.

On Friday, Bello returned to Bronx Lebanon Hospital with an AM-15 assault rifle tucked under his white lab coat and opened fire in his old department, killing one doctor and critically wounding six other people at the hospital, according to law enforcement officials.

Bello then shot himself, and staggered, bleeding, into a hallway where he collapsed and died with the rifle at his side, officials said. A photo showed the doctor on a blood-spattered floor as police stood over him.

Now, detectives are trying to piece together what prompted Bello to snap two years after he was forced out, and whether he was hunting for someone in particular when he went to the 16th floor and started shooting.

“There are many, many details that we’re still putting together,” Mayor Bill de Blasio said, adding that terrorism was not involved in the attack. “This was a horrible situation unfolding in a place that people associated with care and comfort, a situation that came out of nowhere.”