The gunman who fired into a busy In-N-Out Burger in Twin Falls, Idaho, on Saturday, sending customers scurrying for cover in the restaurant and behind cars in the parking lot, was a 24-year-old man who acted alone, the authorities said on Sunday.

At least three people were killed and seven others wounded.

The afternoon rampage, the authorities said, might have been worse if not for the “heroic action” of an off-duty state trooper and a private citizen who returned fire and slowed the gunman.

“We believe their actions helped drive the suspect away from the scene, preventing further casualties,” the Twin Falls police chief, Matthew Hicks, said during a brief news conference Sunday.

Chief Hicks identified the gunman as Chad Williams and said that he died of a self-inflicted gunshot, but offered no other details, including possible motive.

Chief Hicks declined to name the shooting victims because all family members had not yet been notified. Of the seven wounded, two were in critical condition, three were in stable condition and two were treated at a hospital and released. The numbers of the injured, he said, are still considered preliminary.

The chief also confirmed that minutes after law enforcement arrived on the scene and began searching for the gunman, there was a second officer-involved shooting. It was unclear with whom the officer engaged. No one was injured, but the officer’s actions are now under review by the Jerome Police Department.

The primary shooting is being investigated by the Twin Falls Police Department in conjunction with the F.B.I.

“We are still working to determine the full set of circumstances in this incident, including the suspect’s actions and possible motivations,” Chief Hicks said. “The Williams family has been notified, and they are fully cooperating with law enforcement.”

Authorities said they were working to piece together a shooting timeline as well as conducting interviews with hundreds of people who were in and around the shopping center at the time of the shooting.

What is known is that just after gunshots rang out from inside the restaurant, the police received multiple phone calls at about 2:30 p.m. Dispatchers said they could hear screaming during what police described as a chaotic scene, disrupting a typical Saturday afternoon of shopping, movies and dining.

The identities of those killed or injured are slowly emerging, mostly through social media posts. An employee of In-N-Out was among those killed.

“We lost one of our beautiful Associates tonight,” said Lyndi Snyder, the president of In-N-Out Burger in a social media post. “She was taking care of our most important asset — our Customers, and her life was taken by a disgusting human who doesn’t value the lives of others or his own.”

Ms. Snyder did not identify the employee by name.

One bystander, Lane Koehn, 34, said a group gathered around an injured female employee of the restaurant in the parking lot and one man, who said he was a retired paramedic, applied pressure to her chest wound until the ambulance arrived.

“I think it was random, the way the kid was firing,” Mr. Koehn, a construction worker, said in an interview on Saturday. “You can tell he was panicking. He was not very calm and collected, running and slinging his rifle around firing.”

Terry Dudley, a taxi driver in Twin Falls, has also been identified as one of the people shot, according to family members’ social media posts and a co-worker.

Mr. Dudley, 62, was shot while charging his Tesla, a representative of the taxi company said in a GoFundMe post set up to support Mr. Dudley and the other shooting victims.

The post described Mr. Dudley as a widower with four children. “They are his world,” wrote James Palmer, the organizer of the fund-raiser.

“There are very few people in this world like Terry. He is the kind of man who quietly puts others before himself, who works hard, loves deeply, and never hesitates to lend a helping hand,” Mr. Palmer wrote. “He is tenacious, compassionate, kind hearted, and has a way of making those around him smile, even on their hardest days.

Mr. Palmer said Mr. Dudley was the “best employee” he has ever had.

“He is loyal, dependable, selfless, and the kind of person every community is better for having,” he wrote.

Christopher Claunch, who worked at Renaissance Ranch Recovery Center in Rupert, Idaho, died from his injuries, according to his sister, Kimberly Castaneda, of Rosamond, Calif.

A California native, Mr. Claunch had lived in Idaho for about 30 years. Every time he returned to the Pacific Coast, he made sure to stop at In-N-Out, his sister said.

But he told her that when the Twin Falls location opened, he would wait to go, knowing the hype would only cause long waits in line.

“You have no idea how surprised I was when I found out he was there,” she said.

Mr. Claunch had found sobriety two years ago, his sister said, after struggling with alcoholism for most of his adult life.

He lost his first wife in a car crash only after being married for three months, and his sister said he had always hurt from that loss.

In June, the siblings celebrated the Fourth of July in the small town of Hagerman, where he lived.

“My silly brother took my headband that I was wearing with little tin wheels on it,” she said, “and he sat with those things on like it was just no big deal.

“He was always being a goofball,” she said.

Maia Spoto contributed reporting.