People embrace following a funeral for Julia Hyman, associate at Rudin Management Company, one of the victims of Monday's deadly shooting, on Wednesday in New York. (AP photo)

NEW YORK — A man who killed four people at a Manhattan office building bought the rifle he used in the attack and the car he drove across country from his supervisor at a Las Vegas casino, authorities said Wednesday.

Shane Tamura, 27, fatally shot three people Monday in the building lobby before taking an elevator to the 33rd floor, killing a fourth victim and then ending his own life, according to police. The building housed the National Football League’s headquarters and other corporate offices.

In a note found on his body, Tamura assailed the NFL’s handling of concerns about chronic traumatic encephalopathy, and the former high school football player claimed he himself had the degenerative brain disease, according to police. Known as CTE, it has been linked to concussions and other head trauma.

At Tamura’s Las Vegas studio apartment, investigators found a note with a different troubled message, police said Wednesday. They said the note expressed a feeling that his parents were disappointed in him and included an apology to his mother.

Police said they also found a psychiatric medication, an epilepsy drug and an anti-inflammatory that had been prescribed to Tamura.

Investigating his movements as well as his mindset, detectives learned that he purchased the rifle and car from his supervisor at a job in the surveillance department at the Horseshoe Las Vegas, the New York Police Department said.

The supervisor legally bought the AR-15-style rifle he sold to Tamura for $1,400, police said, adding that they had erred in saying earlier that the supervisor supplied only parts of the rifle. It wasn’t immediately clear whether the gun sale was legal.

Police didn’t identify the supervisor, who has been forthcoming with them and hasn’t been charged with any crimes. Tamura had alluded to him, apologetically, in the note found in the gunman’s wallet after the rampage, police said.

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Collins reported from Hartford, Connecticut. Contributing were Associated Press writers Philip Marcelo in East Meadow, New York, and Jim Mustian in New York City.