A gunman opened fire Saturday afternoon at a recently opened In-N-Out Burger in Twin Falls, Idaho, sending a shopping center into chaos as people fled from stores and restaurants. At least three people were killed and five injured, according to the authorities.

The Twin Falls Police Department said it received a call for an active shooter inside the restaurant around 2:30 p.m. The man suspected of the shooting is dead, the authorities said. As of Sunday morning, it remained unclear if the assailant was among the three fatalities or in addition to them.

Josh Palmer, a city spokesman, said the number of victims could increase.

The F.B.I. has set up a digital tip line for the public to submit images and videos related to the shooting.

Here’s what we know:

What happened?

Local and state law enforcement agencies responded to calls of shots fired at In-N-Out Burger around 2:30 p.m. Officers arrived at what the Twin Falls police chief, Matthew Hicks, characterized as “the most chaotic scene that many of them will ever have in their careers.”

The shooting occurred in one of the busiest areas in southern Idaho, Chief Hicks said, in a commercial district with stores such as Petco and Best Buy and restaurant chains like IHOP and Taco Bell.

The authorities are still piecing together the events that led up to the shooting by interviewing hundreds of people who were in the general vicinity.

The In-N-Out location opened on July 24 and was the first in Twin Falls.

The restaurant, known for its burgers, shakes and fries, drew large crowds at its opening, according to local news reports.

How many people were killed?

By Sunday morning, at least three people had been killed and at least five had suffered injuries.

It remains unclear how the gunman died.

Lynsi Snyder, the president of In-N-Out Burger, confirmed that a female worker was among those killed.

“We lost one of our beautiful Associates tonight,” Ms. Snyder wrote on social media. “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.”

What did people see?

In interviews, bystanders depicted a harrowing scene.

Lane Koehn, 34, said he was driving home when he saw what appeared to be a young man in his teens firing a large rifle into traffic near an In-N-Out.

A bystander returned fire with his handgun, which encouraged the young shooter to find cover, Mr. Koehn said.

Mr. Koehn drove into the In-N-Out parking lot, he said, and assisted a group helping the restaurant employee who had been shot in the chest. A man applied pressure to her wound until the ambulance arrived, he added.

Inside the restaurant, Jaylyn Sanchez, 20, barricaded herself in the bathroom after she saw people rushing from the restaurant, “running out of there as fast as they could.”

When she felt the scene was clear, she exited the building, she said. But in the parking lot the gunman pointed his weapon at her. Ms. Sanchez immediately ducked behind a car.

She later rejoined her older brother, Dominic Gonzales, 29, who had been waiting in her truck with their two younger brothers, 5 and 6 years old.

Mr. Gonzales said he heard “pops” after Ms. Sanchez left the truck to enter the restaurant. He told his two younger brothers to get down, he said. They had been near the shopping center to see the new “Spider-Man” movie.