Mike Picard, the research director for GunPolicy.org, which tracks firearm use around the world, said the images and sounds captured by people at the scene pointed to at least six weapons: one or two handguns, including the shooter’s personal firearm, three HK33 assault rifles and two larger M60 machine guns.

The gunman, he said, also appears to have been carrying about 1,000 rounds of ammunition. Local news media reported Sergeant Jakkrapanth was a specialist in long-range sniper fire.

Ms. Viparat, 39, and her husband, Somwang Kwangchaithale, 39, were sitting in a movie theater on the fifth floor of the mall when the lights came on and an emergency announcement came over the loudspeaker around 5:30 p.m. Initially, they stayed in the theater. Then the mall’s staff moved them to an office with a locked door. They huddled together there, 100 of them all together, until around 10 p.m., when the message from the authorities landed: They were about to be evacuated.

“They told us they’re going to turn the lights off, said Mr. Somwang. “‘Stay low and do not make any loud noises.’”

When they reached the basement, the gunman heard them. He started shooting.

“All of the people who gathered at the parking lot started screaming and running for their lives,” said Ms. Viparat. “Rescuers helped us out. Police, rescuers, military, different officers.”

By that time, the authorities had launched into full operation mode. Shortly after 8 p.m., the police declared the gunman a most-wanted person and urged the public to call in tips, presenting a photo that showed him looking bored, with indifferent eyes.

They also started moving large numbers of people out. They urged evacuees to “raise their hands” and identify themselves. They were wary that the gunman was hiding in the crowd.