A New York Police officer stands guard on the roof of a building across the street from the Bronx Lebanon Hospital in New York. Police say a fired physician returned to the hospital and opened fire.

NEW YORK — Dr. Henry Bello proved a man of his word.

After he was forced to resign as a family medicine doctor amid sexual harassment allegations, he threatened his colleagues. He said he would kill them.

On Friday, Bello returned to Bronx Lebanon Hospital with an AR-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.

“There are many, many details that we’re still putting together,” said 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.”

His former co-workers described a man who was aggressive, loud, and threatening.

“All the time he was a problem,” said Dr. David Lazala, who trained Bello as a family medicine doctor. When Bello was forced out in 2015, he sent Lazala an email blaming him for the dismissal.

“We fired him because he was kind of crazy,” Dr. Maureen Kwankam told the Daily News. “He promised to come back and kill us then.”

People described a chaotic scene as gunfire erupted, spreading terror throughout the medical facility as employees locked themselves inside rooms and patients feared for their lives after hearing an announcement warning of someone in the building with a weapon.

“I thought I was going to die,” said Renaldo Del Villar, a patient who was in the third-floor emergency room getting treatment for a lower back injury.

Shortly after receiving a 911 call about an active shooter, police officers went floor by floor, their guns drawn, looking for the gunman. Fifteen minutes later they confirmed he was dead in the building.

Bello may have doused himself with an accelerant like gasoline and tried to set himself on fire before shooting himself, officials said. Sprinklers extinguished the fire.

The officials were not authorized to discuss the still-unfolding investigation and spoke to The Associated Press on condition of anonymity.

According to New York State Education Department records, Bello graduated from Ross University and had a limited permit to practice as an international medical graduate to gain experience in order to be licensed. The permit was issued on July 1, 2014, and expired last year on the same day. Family medicine doctors handle more routine cases, such as coughs and sprained ankles.

Bello also worked as a pharmacy technician at Metropolitan hospital in Manhattan because he was having a hard time getting licensed as a physician, but quit the job in 2012 and filed for unemployment, according to the lawyer who represented him on appeal in 2014. He lost his case.