LOS ANGELES (CBSLA.com/AP) — Police are looking for an assailant who they say struck and killed a 45-year-old La Puente man with a single punch during a brief confrontation in downtown Las Vegas.

Officer Jacinto Rivera said Friday that investigators are trying to identify a man seen on police security video walking and then running away with another man following the attack early Sunday on Fremont Street.

The Clark County coroner says Luis Campos died Thursday at a Las Vegas hospital.

According to police, Campos was standing in a line to enter the nightclub when he was approached by at least two men. After a brief verbal altercation, one of the suspects punched the victim, and both suspects fled the scene on foot.

Campos’ brother, Drake Garibay, told the Las Vegas Review-Journal they were standing in line outside a club when the attacker stopped while walking past on the sidewalk, asked what Campos was looking at, and punched him.

Garibay said his brother never regained consciousness. He died Thursday morning.

“I’m in shock,” said his mom, “And now I just want to catch this guy.”

A GoFundMe page set up for the family had raised nearly $9,000 by Friday.

“He loved his wife, he was a hard-working man,” says his mother, “He made time for everybody, dependable. Everybody loved him. He was my best friend.”

Detectives released video footage of two men believed to be the assailants walking away from the scene. They were described as Hispanic males in their 20s. One suspect was seen wearing a white cap, dark shirt and white pants, and the other was wearing a dark shirt and tan pants.

(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)