Police did not immediately make details of the shooting public, but a law enforcement source told Reuters that a New York Police Department officer was killed, two civilians were struck by gunfire and the suspect was dead.
The source said the information was preliminary, and the condition or extent of injuries sustained by the two civilians was not immediately known.
Other news media outlets, citing their own sources, reported death tolls between four and five, not including the suspect.
New York Mayor Eric Adams said in a video message posted on X there were "multiple injuries" in the shooting.
A short time later, New York Police Commissioner Jessica Tisch confirmed on X: "At this time, the scene has been contained and the lone shooter has been neutralised."
The New York Post newspaper, citing law enforcement sources, reported a gunman wearing a bullet-resistant vest and carrying an AR-style rifle had opened fire inside the Park Avenue skyscraper, killing an NYPD officer, before turning the weapon on himself.
At one point during the violence, the gunman had barricaded himself inside the office tower, possibly on the 32nd floor of the building, according to the Post.
Several news outlets, including CNN, the New York Post and NBC News, said the suspect was tentatively identified as a 27-year-old man from Las Vegas.
CNN said police shared a photo of the suspect walking into the building carrying a rifle. 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 a number of financial firms, including Blackstone, KPMG Deutsche Bank and Morgan Stanley, 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, who was working out in a gym next to the skyscraper, told Reuters near the scene.
The FBI said agents from its New York field office were also responding to provide support at the scene.
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