Investigators are confident the gunman, identified as 24-year-old Chad Williams, acted alone during the shooting on Saturday in Twin Falls, Police Chief Matthew Hicks said at a news conference.
Three people died and seven others were injured at the recently opened restaurant in the southern Idaho city of about 56,000 people.
Police are still working to determine Williams' motivations and his movements leading up to the shooting in a shopping area, Hicks said.
Hicks said an off-duty state trooper and an armed citizen - whose identities weren't immediately disclosed - both fired back and diverted the gunman away from the restaurant.
"We believe their actions helped drive the suspect away from the scene, preventing further casualties," the chief said.
"We want to take a moment and commend the actions of the off-duty officer and the citizen, and their heroic action to stop this incident."
Hicks said detectives are working to determine Williams' motivations, and said the suspect's family was co-operating fully with law enforcement.
One of those killed was an employee, the fast-food chain's owner said.
"My heart is broken, my chest is heavy, my eyes burn from the tears I've shed. We lost one of our beautiful Associates tonight," In-N-Out owner Lynsi Snyder said in a statement released early Sunday on social media.
"In-N-Out will be mourning with the families and praying for them as they tread through a very painful season."
The FBI on Sunday encouraged the public to submit images and videos related to the shooting Saturday in Twin Falls that left seven other people wounded, some critically.
The In-N-Out had opened just about a week earlier, on July 24, as part of the California-based company's recent expansion to Idaho.
with AP
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