The 1 p.m. attack at a Chevron station just off Interstate 980 near downtown Oakland was recorded on surveillance video and showed three assailants exit a black sedan stopped at a gas pump and confront Joyner as he stood at the adjoining pump putting gas in a white Porsche.

That one-minute video captured “the violence that we’ve seen in the city this entire year,” Oakland Police Chief LeRonne Armstrong told reporters who gathered outside Highland Hospital, where Joyner was “fighting for his life” hours after he was shot, Armstrong said.

What Armstrong described as a “brazen” daylight assault on a police veteran happened in a city that has struggled to curb its homicide rate as residents and politicians battle over how to reform policing amid calls to beef up social service programs to deter crime.

Police found Joyner after responding to a ShotSpotter alert shortly after 1 p.m. at the Chevron gas station on 1700 Castro Street, the edge of the Uptown district.

Surveillance video obtained by The Chronicle showed the three assailants going through Joyner’s pockets and removing items from him before they stepped away from him and opened two doors of the Porsche. At that moment Joyner stepped toward the back of the Porsche, pulled out a handgun he was carrying and opened fire — appearing to shoot two of the assailants, who were only steps away. One ran toward the black sedan and the other crumpled to the ground.

Joyner, with his gun still drawn, then walked around the gas pump to approach the two other assailants as they began to speed off. Joyner immediately recoiled, apparently shot in the torso, and stumbled back toward his car, tripped over the body of the downed assailant and was tangled in the gas hose.

One of the assailants then stepped out of the sedan and tried to pull the person who had been shot by Joyner away but gave up the attempt and fled.

Two hours later, law enforcement officers were searching the ground around the gas pumps, which had been littered with bullet casings when they arrived, according to two sources.

A couple of officers rolled a gurney into the Alameda County coroner’s truck while others continued inspecting the area for evidence. They had closed off the surrounding blocks with caution tape and patrol vehicles as a police helicopter circled overhead. Officers had parked a tactical unit van near the gas pumps.

Addressing the scrum of reporters at Highland Hospital, Armstrong did not name Joyner, but confirmed that the victim was a retired police officer and that police are looking for a black sedan. Multiple law enforcement sources confirmed Joyner’s identity.

Police said the victim was in critical but stable condition Thursday night. No arrests had been made. The retired officer was “lawfully permitted to carry a gun,” Armstrong said.

Joyner, who was born and raised in East Oakland, joined the Oakland Police Department in 1991, according to his LinkedIn profile, after studying criminal justice at Cal State East Bay. Known as an aggressive and ambitious officer, he had been promoted to lieutenant by 2006,

Over the course of his career Joyner was involved in several controversies, including a 2011 fatal shooting that ended with the city paying a $75,000 settlement five years later. By then, Joyner had become a high-profile leader in the police force, taking the helm of the Ceasefire anti-violence program in 2013 and overseeing its expansion.

During the six years that Joyner steered Ceasefire, homicides in Oakland declined significantly. Toward the end of that span, the number of annual shootings and homicides was about half what it was in 2012.

When he left the Police Department, Joyner started a new career in the cannabis industry. He now owns dispensaries in Oakland.

Armstrong said that at this point investigators have no reason to believe that Joyner was targeted specifically because of his past positions in law enforcement or his current ties to the cannabis industry. However, the chief said, “at this point we will be looking into everything.”

Under California law, a person can claim self-defense in a killing if they reasonably believed they or someone else were in imminent danger of being slain or badly injured, or that they were in danger of becoming the victim of forcible crimes such as robbery or rape. The person must have a reasonable belief that their use of deadly force is necessary. But they are not required to retreat, the law states, and can pursue an assailant until the danger has passed.

Police have investigated 115 homicides in Oakland so far this year.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan