A gunman opened fire on Monday inside a Midtown Manhattan skyscraper housing NFL headquarters and offices of several financial firms, including Blackstone, killing four people before fatally shooting himself, New York City officials said.

One of the four victims slain in the gun violence was a 36-year-old New York Police Department officer who had been on the force for about 3 1/2 years. The three others killed by the suspect were civilians.

New York Police Commissioner Jessica Tisch said the gunman, who resided in Las Vegas and drove cross-country to New York in recent days, fatally shot himself in the chest at the end of his shooting spree.

Tisch said the gunman was believed to have acted alone, and investigators had yet to determine a possible motive for the shooting.

A photo of the suspect that CNN said was shared by police showing a gunman walking into the building carrying a rifle was published by a number of major news media outlets. Preliminary checks of the suspect's background did not show a significant criminal history, the report added, citing officials.

The skyscraper at 345 Park Avenue houses offices of a number of financial institutions, including Blackstone and KPMG, along with the NFL headquarters.

A large police presence converged on the area around the tower, according to Reuters journalists near the scene.

"I just saw a lot of commotion and cops and people screaming," said Russ McGee, a 31-year-old sports bettor who was working out in a gym adjacent to the skyscraper, told Reuters in an interview near the scene.

The FBI said agents from its New York field office were also responding to provide support at the scene.