A 26-year-old coder wrested an assault pistol from the gunman’s hands, preventing a greater tragedy.

His older sister, Brenda, who currently runs the business, said the video showed a prolonged, fierce struggle between the two men all over the lobby.

“He kept coming at him, he really wanted the gun back,” she said of the gunman.

The younger Mr. Tsay, a computer coder who mans the ticket office a few days a week at the ballroom started by his grandparents, said it was around 10:35 p.m. Saturday that he turned to face the gunman, whom he didn’t recognize. He had never seen a real gun before, but could tell that it was a deadly weapon, he said.

“My heart sank, I knew I was going to die,” he said.

The next moment, he lunged and grabbed the weapon by its barrel and began wrestling with the gunman for control of it.

“That moment, it was primal instinct,” he said. “Something happened there. I don’t know what came over me.”

They fought over control of the gun for about a minute and a half, and it felt like they were similarly matched in strength, Mr. Tsay said. At one point, the gunman looked down at the weapon and took one hand off it, as if to manipulate the gun to begin shooting. Mr. Tsay said he seized the moment and pried the pistol away from the man.

He pointed the weapon back at him and yelled: “Go, get the hell out of here,” he recalled.

Mr. Tsay, who stayed up all night assisting police with their investigation, said he felt traumatized and hadn’t quite been able to process what he had been through. He particularly felt heartbroken for the community of Monterey Park and surrounding areas where his family and their ballroom had become established as a beloved haven over three decades, he said.

“Lai Lai,” a name his grandmother chose, means “come, come,” in Chinese, his sister said. The assailant, dressed in black, looked like he could easily be one of their regulars, he said.

“We have such a tight-knit community of dancers,” he said. “It feels so terrible something like this happened, to have one of our individuals try to harm others.”

Shawn Hubler contributed reporting.