A man carrying a rifle killed four people, including an off-duty police officer, in a New York office building that houses some of the country’s top financial firms and the National Football League (NFL), officials said.

Police identified the gunman as Shane Tamura, of Las Vegas, and said he also wounded a fifth person before taking his own life.

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He had a “documented mental health history”, but the motive was still unknown, Police Commissioner Jessica Tisch said at a news conference.

“We are working to understand why he targeted this particular location,” she said.

The officer who was killed was a 36-year-old immigrant from Bangladesh and had served as a police officer in New York City for three and a half years.

“He died as he lived. A hero,” Ms Tisch said.

He was married with two young boys, and his wife was pregnant with their third child, she said.

“He was saving lives, he was protecting New Yorkers,” Mayor Eric Adams said.

A night-time photo showing an emergency services worker pushing a woman in a wheeled chair.

The gunman’s motive remains unknown, officials say. (Reuters: Eduardo Munoz)

“He loved this city, and everyone we spoke with stated he was a person of faith and a person that believed in God.”

Mayor Adams said the man who was injured remained in a critical condition.

He added that officials were still “unravelling” what took place.

“Five innocent people were shot,” he said.

Gunman ‘sprayed’ lobby

Surveillance video showed a man exiting a double-parked BMW with an M4 rifle before he walked toward the building.

He immediately opened fire on the NYPD officer as he entered the building, shot a woman who tried to take cover, then began “spraying” the lobby with gunfire, Commissioner Tisch said.

The man then made his way to the elevator bank and shot a security guard who was taking cover behind a security desk, and another man in the lobby, the commissioner said.

Officers found a rifle case, a revolver, magazines and ammunition in his car, she added.

The Fire Department of New York said emergency crews were called to the Park Avenue office building following a report someone had been shot.

A spokesperson said the call came in at about 6:30pm on Monday, local time, but provided no other details.

Ambulance stretchers sit on the street in front of a New York ambulance.

Stretchers are prepared as emergency services gather in Manhattan. (Reuters: Jeenah Moon)

‘Multiple shots in quick succession’

Jessica Chen told ABC News US she was watching a presentation with dozens of other people on the second floor when she “heard multiple shots go off in quick succession from the first floor”.

She and others ran into a conference room and barricaded tables against the door.

“We were honestly really, really scared,” she said, adding that she texted her parents to tell them that she loved them.

Mayor Adams posted a clip to social media urging people still inside the office building to stay put because police officers were conducting a floor-by-floor search.

The Democrat said he was headed to the hospital to speak with the family and loved ones of those involved.

Local TV footage showed lines of people with their hands above their heads evacuating the office building.

Finance workers at an office building down the block were picking up dinner at a corner eatery when they heard a loud noise and saw people running, AP reported.

“It was like a crowd panic,” said Anna Smith, who joined the workers pouring back into the finance office building.

They remained there for about 2 hours before being told they could leave.

The building includes offices for Blackstone and the Consulate General of Ireland, located a short walk north from Grand Central Terminal.

A press conference with many sober-looking people and a black man in an NYPD cap speaking at a wooden lectern.

New York Mayor Eric Adams (centre) addresses members of the news media, alongside NYPD commissioner Jessica Tisch (right). (Reuters: Bing Guan)

The city’s emergency management alert system warned of traffic delays, road closures and disruptions to public transportation in the area.

Through late July, New York City is on pace to have its fewest murders and fewest people hurt by gunfire than any year in recent decades.

AP