TWIN FALLS, Idaho (AP) — Investigators on Monday were trying to determine what led a 24-year-old man to open fire at an Idaho fast-food restaurant over the weekend, in an attack that left three people dead and seven others wounded before authorities say he killed himself.

Police Chief Matthew Hicks said Sunday detectives are confident that the gunman, identified as Chad Williams, acted alone in the shooting Saturday at an In-N-Out Burger in Twin Falls, a southern Idaho city of about 56,000 people.

He said Williams’ family is cooperating fully with the investigation.

An off-duty state trooper and a civilian fired on the attacker and diverted him from the restaurant, the chief said. Police haven’t disclosed their names or those of the victims. Twin Falls spokesperson Josh Palmer said Monday that police are still trying to identify next of kin.

Two of the wounded were treated at a hospital and released, three were listed as stable, and two others were in critical condition, police said Sunday.

An In-N-Out employee was among those killed, the fast-food chain’s owner, Lynsi Snyder, said in a statement. She didn’t identify the worker.

The restaurant, which opened July 24 as part of the California chain’s Idaho expansion, is in a busy shopping plaza, and hundreds of people were in the area when the shooting happened.

Lane Koehn, 34, said he was at a nearby stoplight when he saw someone with an AR-style rifle emerge from the drive-thru. A man with a pistol started firing at the shooter.

He then saw someone in an In-N-Out uniform drag a person, also wearing a uniform, bleeding from a gunshot wound to her chest across the parking lot. The employee, Koehn and the man with the pistol stayed until paramedics arrived for her.

“She was pretty bad off, but I don’t know. I hope she made it,” Koehn said.

He said the man holding the rifle fired at least three or four rounds, though it wasn’t clear if anyone in particular was targeted.

Video taken by a driver from across the street captured a shooter, who appeared to be dressed in black, walking up to a white car carrying a long rifle, opening the driver-side door and popping the trunk. The person then walked to the trunk and reached inside. At that point, the driver who shot the video left quickly, fearing that the man might be getting more weapons.

Nayeli Rodriguez, 40, of Burley, was inside waiting for food with her daughters when she heard shots from the kitchen. An employee said it was a shooting and told people to get down. Rodriguez said an older man tried to protect her and her daughters, and they were among the last to run outside.

“We were trying to get as far as we could and some angels let us in their car,” Rodriguez said via text message. “I left my car there in the parking lot.”