New York City police detectives searched a studio apartment in Las Vegas that belonged to the suspected gunman in Monday's mass shooting at a Midtown Manhattan.

Tamura had apparently driven across the country from Nevada before killing four people in a shooting rampage in Manhattan.

In the aftermath of New York’s deadliest mass shooting since 2000, authorities are investigating the motives and background of the man who opened fire in a midtown Manhattan office skyscraper, killing four people and then himself.

Four people were killed, including an off-duty New York City police officer, when a gunman opened fire in an office tower before fatally shooting himself, officials said.

The attack on a seemingly secure building has rattled workers and prompted managers to examine whether they are adequately protected.

The man who opened fire in a Midtown Manhattan office building, killing four people and wounding a fifth, appeared to be targeting the NFL headquarters and had a note claiming he suffered from CTE, police sources told ABC News.

The gunman who killed four people in a midtown Manhattan office building left a suicide note indicating that he was targeting the NFL, police said.

The head of the New York Police Department said that investigators plan to question a man who supplied parts for a gun used in the attack on Monday.

Four victims, including an NYPD officer and a Blackstone executive, were killed in an NYC shooting at 345 Park Avenue.