A New York police officer, a “beloved” security guard, a young Cornell graduate and a senior executive with investment firm Blackstone were killed Monday when a lone gunman stormed a sprawling office tower in Midtown Manhattan and opened fire.

The alleged gunman at a shooting rampage in a midtown Manhattan office building was identified as 27-year-old Shane Tamura, of Nevada.

The New York Yankees held "a moment of silence in memoriam of NYPD Officer Didarul Islam" before Tuesday's game. The 36-year-old served on the force four years.

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.

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,