The gunman who opened fire in a New York City skyscraper, killing four people, purchased the rifle used in the attack from his boss at a Las Vegas casino, investigators said.
The supervisor, who has not been named, is co-operating with officials, and has not been charged. It is not known whether the sale was legal.
Shane Tamura, 27, died by suicide and left a note claiming to be suffering from CTE, a brain disease triggered by head injuries, police said.
The attack took the lives of policeman Didarul Islam, executive Wesley LePatner, security guard Aland Etienne, and real estate worker Julia Hyman.
Police said on Wednesday the semi-automatic rifle - also known as an assault rifle - was sold by the Horseshoe casino supervisor for $1,400 (£1,100). Tamura worked for the surveillance team at the casino.
