The AR-15-style weapon used by 27-year-old Shane Devon Tamura in Midtown Manhattan yesterday was assembled using a lower receiver purchased by an associate of Tamura’s, New York City Police Commissioner Jessica Tisch said today.

The lower receiver is the part of an AR-15 that is regulated by federal code. Purchasing the part requires a background check in Tamura’s home state of Nevada with limited exceptions to weapon transfer between family members. Since 2020, both the buyer and the seller involved in a private sale that bypasses Nevada’s background check laws can potentially face criminal penalties.

Investigators have interviewed “Rick,” the individual who, Tamura apologizes to in the note found in his back pocket, a law enforcement official told CNN. Investigators believe Rick is the same person who bought the lower receiver for Tamura and is a former coworker of his at one of his security jobs, the law enforcement official added.

Two groups of detectives are enroute to Las Vegas where they will conduct interviews and execute a search warrant at Tamura’s home, the commissioner said. Detectives will also visit the gun store where Tisch says Tamura legally purchased a revolver on June 12.

Tamura had two cell phones, which officials will also search, Tisch added.

As police continue to investigate Tamura’s background, an NYPD official added that authorities had determined that Tamura had no connection to the NFL.

A source told CNN a suicide note found in Tamura’s pocket expressed grievances with the NFL and said he suffered from CTE, a disease linked to head trauma.

Part of the note read, “study my brain, I’m sorry,” Tisch said.

This post has been updated with additional information.