No motive has been announced. A suicide note was found expressing grievances with the NFL and saying Tamura suffered from CTE, a source said. Follow for live updates.

The shooter was Shane Tamura; authorities say he was intending to target the NFL offices in Manhattan but entered the wrong elevator in the Midtown skyscraper.

But on Monday evening, Islam, who was regularly assigned to the 47th Precinct in the Bronx, was gunned down by an armed suspect identified as Shane Tamura, as the officer was dong a paid detail shift at 34d Park Avenue, a skyscraper home to several large New York City corporations.

Part of the gun used by a Las Vegas man to kill four people last night in a Midtown Manhattan office building is under investigation as being provided by an associate. NBC News National Law Enforcement and Intelligence Correspondent Tom Winter breaks it all down.

Authorities also found a vehicle with Nevada plates registered to Tamura at the scene, with a rifle case inside, sources said.

Two teams NYPD detectives headed to Las Vegas Tuesday, to execute search warrants at Shane Tamura's home and at a gun store where he legally purchased a revolver on June 12 using a Nevada concealed carry permit,

After four people were killed last night in a Midtown Manhattan office building by a lone gunman, safety concerns for the future are top of mind. Teneo Risk Advisory Executive Chairman and former NYPD Chief Bill Bratton joins Erielle Reshef to discuss the steps that should be taken to combat attacks.

Tamura, 27, said in the note, which he had on him when he died and is a few pages long, that he wants his brain to be studied, the sources said.