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.

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.