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.