A disgruntled doctor armed with an assault rifle and wearing a lab coat went on a rampage in the Bronx hospital where he had worked on Friday, killing a female doctor and wounding six other people — five of them seriously — before setting himself on fire and shooting himself in the head, the authorities said.

The furious attack by the doctor — identified by the police as Henry Bello, 45 — sent workers at the hospital, Bronx-Lebanon, diving behind desks and doors as gunshots and smoke filled the hallways of a place devoted to healing. Witnesses described medical workers’ ripping a fire hose from the wall to use as a tourniquet on one victim’s leg, while others recalled the horrific sight of the gunman, his torso aflame, running down a hallway.

Terrorism was quickly ruled out. Instead, it appeared to be the type of mass shooting by a lone gunman that has struck communities around the United States.

“He’s shooting! He’s shooting!” one woman yelled in the frantic initial moments of the afternoon assault, as recounted by a mother in the pediatric emergency room who had cowered with her five children, ages 1 to 10.

Some believed that the death toll would have been far higher had the shooting occurred anywhere but where it did — a hospital filled with state-of-the-art medical equipment, and with doctors and nurses who rushed to victims even as the gunman was still at large and performed triage where they fell, in staircases and hallways.

“The situation unfolded in the middle of a place that people associate with care and comfort,” Mayor Bill de Blasio said during a news conference outside the hospital on the Grand Concourse in the Claremont Village neighborhood. The gunman acted alone, Mr. de Blasio said, adding that it appeared to be a workplace dispute that ended when the gunman committed suicide — “but not before having done horrible damage,” the mayor said.

The police did not identify the victim, other than as a woman and a doctor. The five seriously injured patients were “fighting for their lives,” said the police commissioner, James P. O’Neill. The sixth had a gunshot wound to the leg.

Dr. Bello was going to be fired by the hospital, but instead chose to resign, after he was accused of sexual harassment, said a police official, who was granted anonymity to discuss a continuing investigation. Another police official said Dr. Bello had left the hospital some time ago. “He didn’t leave voluntarily,” the official said.

Dr. Bello was armed with a rifle, an AR-15, that investigators believed he sneaked into the hospital under his lab coat, the two police officials said.

While investigators were still trying to determine a motive, one official said, “Most likely it’s a workplace violence on the part of a former disgruntled employee.”

The New York State Education Department said that Dr. Henry Bello had received a limited permit to practice as an international medical graduate in order to gain experience so he could be licensed. The permit was issued on July 1, 2014, and expired on July 1, 2016. He also had a pharmacy technician license that had been issued in California in 2006.

Shortly after the 2:50 p.m. shooting, graphic images emerged online that purported to show the suspect lying on the hospital floor, in a button-down shirt worn under a lab coat, the scene covered in blood.

In 2004, Dr. Bello was arrested and charged with sex abuse and unlawful imprisonment after a 23-year-old woman told officers he had grabbed her crotch area outside a building on Bleecker Street in Manhattan and tried to penetrate her through her underwear, a law enforcement official said. The woman told officers that Dr. Bello had lifted her up in the air and dragged her while saying, “You’re coming with me.”

Court records indicate that in September 2004, Dr. Bello pleaded guilty to unlawful imprisonment in the second degree, a misdemeanor, and was sentenced to community service; the felony sexual abuse charge was dismissed.

Norma Ruiz, a patient-care technician at Bronx Lebanon, said the shooting took place on a floor where she works, and recalled seeing a man, now believed to be the gunman, on fire. “He was running down the hall throwing himself on the floor,” she said. “We throw ourselves on the floor, and when everybody was quiet, my co-worker and me, we lift our heads and the doctors’ station was on fire.”

The carnage took place on the 16th and 17th floors of one of the Bronx’s largest hospitals, which sees 1.1 million patient visits and over 140,000 emergency room visits a year, according to its website. Witnesses said that on Friday afternoon, Bronx-Lebanon’s rooms and corridors were filled with patients and visitors.

“Everybody just started screaming,” said the mother in the pediatric unit, who asked that her name not be used to protect her children’s medical privacy. The hospital staff frantically tried to quiet everyone, she said, ordering people in the packed waiting room to lie on the floor while the lights were turned off.

The woman ran into an exam room with her children and shoved them underneath a hospital bed, while she lay on the floor as a barricade in front of them. The ordeal lasted an hour. When her 1-year-old began to cry, she breast fed him to keep him quiet, she said. “My heart was pounding,” she said. “I was shaking, just shaking.”

As the situation unfolded on Friday afternoon, emergency workers were at first prevented from entering the building. At least one of the wounded was being treated by people inside the hospital who had tied an emergency fire hose as a tourniquet, a Fire Department official said. At one point, the police escorted into the building a group of emergency workers wearing armor, even as the gunman was still being sought.

Inside the pediatric unit, while the gunman was still at large, a woman answering the phone, who declined to give her name, said that she and others were hunkered down, unable to leave. “We are just keeping safe. We are O.K.,” she said. “All of us have to feel tense because of the situation.”

Miguel Mercado, 61, was lying in a hospital bed on the 10th floor, waiting for an M.R.I. after complications from back surgery earlier in June, when the police burst into the room and ordered patients who could walk to head down the stairs with their hands up. “The cops came and started emptying out the rooms floor by floor — ‘Everybody get out, get out, get out!’” he said about an hour after the shooting, standing in the parking lot outside the hospital, IV tubes dangling from his arm. On his feet he wore only hospital socks.

“It’s been happening almost anywhere, but nobody was waiting for this,” Mr. Mercado said, referring to the workplace shooting. “Who would have thought it would happen in a hospital?”

As patients and employees drifted from the hospital in the hours after the shooting, Ms. Ruiz, the patient-care coordinator, stood in her green scrubs, deeply shaken. “I just want to get out of here,” she said, recalling the moment she heard the gunman. “We hear, ‘Boom, boom, boom.’ We thought, ‘A patient, a relative.’ But no, it came from the doctors’ station.”

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