A gunman who killed four people including a police officer when he stormed a Manhattan office tower and opened fire, before turning the gun on himself, has been identified by officials.

Police say Las Vegas resident Shane Devon Tamura, 27, sparked utter chaos and panic on Monday when he walked into a 44-storey building that houses Blackstone and the NFL headquarters armed with an assault rifle and began shooting about 6.30pm.

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Among the victims were New York City police officer Didarul Islam, a 36-year-old father of two preparing to welcome a third child with his wife.

“(Islam) put himself in harm’s way. He made the ultimate sacrifice — shot in cold blood, wearing a uniform that stood for the promise that he made to this city,” New York Police Commissioner Jessica Tisch said.

Three civilians — including a woman who reportedly took cover behind a pillar and a security guard who had hidden behind a desk — were also killed.

Another person was critically injured and is fighting for life.

The rampage came to an end when Tamura shot himself in the chest.

Police believe the shooter acted alone and there “is no longer an active threat to the public”.

Authorities are still trying to work out a motive. They said Tamura “has a documented mental health history”.

“His motives are still under investigation and we are working to understand why he targeted this particular location,” Tisch said.

“345 Park Avenue is a commercial office building whose tenants include the NFL, Rudin Management, KPMG and Blackstone.”

The gunman drove cross-country and arrived in New York City only a short time before the shooting.

His car has been searched.

“Inside of that vehicle, officers found a rifle case with rounds, a loaded revolver, ammunition and magazines, a backpack, and medication prescribed to Mr Tamura,” Tisch said.

A former high school classmate of Tamura said he was “the biggest goofball in the world”.

“Just a ton of energy, class clown,” Caleb Clarke told NBC.

“Just one of those guys, he’s got so much going for him. Just figure out the whole school thing, and he could be somebody great.”

Clarke said “you never would have thought violence was something you’d associate with him”.

“Everything he said was a joke,” he said.

“The only thing I can really think of is there was a point where it looked like the sky was the limit, and then it wasn’t anymore.”

KPMG expressed the firm’s condolences to the victims and their families.

“Our hearts go out to the victims of this horrific act and their families,” it said in a statement.

“We are incredibly grateful for the bravery of building security and law enforcement.”