A veteran Los Angeles police sergeant and a motorist in his 30s whose truck broke down were killed early Monday after the sergeant stopped on the southbound 405 Freeway in the Brentwood area to assist the man and both were struck by another vehicle.

The crash shut down the southbound 405 for the morning rush hour.

Los Angeles Mayor Karen Bass identified the sergeant as Shiou Deng, 53, a 27-year Los Angeles Police Department veteran.

“The men and women of LAPD put their lives on the line for Angelenos each and every day,” Bass said. “Sgt. Deng served L.A. for more than 25 years. His last act was one of service — putting the safety and well-being of others above all else. … My thoughts are with Sgt. Deng’s family during this devastating time.”

The crash was reported to the California Highway Patrol at 1:58 a.m. just north of Moraga Drive where a silver truck was blocking the HOV lane.

LAPD Chief Jim McDonnell said the sergeant came upon the disabled vehicle and stopped, activating the rear lights on his patrol car to ward off traffic. The sergeant got out to assist the other motorist, when both were struck by another vehicle.

The motorist died at the scene, Los Angeles Fire Department spokeswoman Lyndsey Lantz said. The sergeant was taken by ambulance to Ronald Reagan UCLA Medical Center, where he was pronounced dead.

McDonnell, Bass and a host of LAPD officers descended on the hospital.

The chief hailed Deng as “exceptionally funny” and “caring to a fault” to the officers under his command. McDonnell said Deng “tried to make the situation we face as good as it can be for his officers and for the people we have the privilege to serve in our society.”

McDonnell said Deng was a “true leader. He died a hero. He was out there caring for others, putting their safety before his own.”

Flags above city buildings will be lowered to half-staff in Deng’s honor, Bass said.

Around 9 a.m. Monday, dozens of LAPD officers assembled at the Westwood hospital to take part in a procession escorting Deng’s body to the medical examiner’s office in East Los Angeles. Gov. Gavin Newsom offered condolences, noting Deng’s 17 years with the LAPD’s Mental Evaluation Unit.

“We join his family, friends, and fellow officers in mourning, and in honoring his memory,” Newsom said. “May his service never be forgotten.”

The governor announced that flags at the State Capitol would also be lowered to half-staff to honor Deng, who is survived by his wife and parents.