Update 2:45 p.m. ET March 23: Court documents showed that the man accused of killing 10 in a shooting Monday at a King Soopers in Boulder purchased an assault rifle less than a week before the attack.

Supermarket employees told investigators that Ahmad Al Aliwi Alissa shot an elderly man multiple times outside the Boulder grocery store before going inside, according to the documents. Another person was found shot in a vehicle next to a car registered to suspect’s brother. The documents did not say where the gun was purchased.

Authorities said Alissa was from the Denver suburb of Arvada and that he engaged in a shootout with police Monday afternoon inside the store. The suspect was being treated at a hospital and was expected to be booked into the county jail later in the day on murder charges.

Investigators have not established a motive, but authorities believe he was the only shooter, Boulder County District Attorney Michael Dougherty said.

A law enforcement official briefed on the shooting told The Associated Press that the gunman used an AR-15 rifle, a lightweight semiautomatic rifle. Officials were trying to trace the weapon. The official was not authorized to speak publicly and spoke to AP on condition of anonymity.

The suspect’s family told investigators they believed Alissa was suffering some type of mental illness, including delusions. Relatives described times when Alissa told them people were following or chasing him, which they said may have contributed to the violence, the official told AP.

Boulder supermarket shooting