Two impulses seemed to tug at Ersie Joyner across the trajectory of his career in the Oakland Police Department: a determination to hunt down perpetrators, and an altruistic quest to help his community.

The retired police captain, 52, was critically injured in a shootout with three assailants who robbed him Thursday, in the heart of a city where he grew up wary of the police, only to become a decorated law enforcement officer who would steer youth away from violence.

He remained hospitalized at Highland Hospital on Saturday as police continued investigating the incident at a Chevron Station just off Interstate 980 near downtown Oakland. Surveillance video captured a harrowing scene: Three assailants ambushed and robbed Joyner as he pumped gas into his Porsche; Joyner pulled out a gun and shot two of the attackers — one fatally — before he was shot as the two jumped in their car and fled.

Police have not made arrests in this case or identified the assailants.

In a city jolted by gun violence, the incident stunned those who knew Joyner as a cop who had been so instrumental in fighting crime and counseling youth. Suddenly he had become a crime victim himself.

Born and raised in East Oakland, Joyner joined the police force in 1991 with notions that he would change the corrosive relationship between officers and the communities they served, said Regina Jackson, CEO of the East Oakland Youth Development Center and president of the city’s Police Commission.

“He told me the reason he became a police officer was because he didn’t like police officers,” Jackson said. “He wanted to be the one who was more engaged and made people not dislike police officers.”

Joyner had a mission to set youth “on the right track,” Jackson said, and helped mentor teenagers at the East Oakland center and similar organizations. But he was also known for being aggressive and ambitious.

After starting out as an undercover narcotics officer, Joyner eventually switched to the SWAT team. He cannily worked sources for information, including the location of parolee and rape suspect Lovelle Mixon, who killed four Oakland police officers and then died in a shootout with police in 2009.

“He’s from Oakland, he’s well known in Oakland — that brought a lot of credibility,” said David Muhammad, a former Alameda County chief probation officer and criminal justice reform consultant who has worked closely with Joyner for a decade.

At the same time, Muhammad recalls Joyner voicing regrets about his actions as a beat cop in the early 1990s. Mostly, Muhammad said, he had misgivings about the number of arrests he made as part of the so-called war on drugs.

He rose up the ranks, becoming a lieutenant in 2006 and overseeing the homicide unit, where, according to a biography on his booking website for public speaking gigs, he supervised 373 homicide investigations and 30 shootings by police officers.

From there, Joyner continued his ascent, promoted to commander in the Bureau of Field Operations in 2009 and then to captain in 2010. He came under scrutiny the next year, after he and another officer fatally shot two men they believed were headed to commit a murder. Prosecutors cleared the officers of potential charges, and the families sued Oakland for wrongful death, leading the city to pay a $75,000 settlement in 2016.

Several law enforcement and city sources have said Joyner had a fraught relationship with court monitor Robert Warshaw, who for years has overseen mandatory reforms in the city’s Police Department, stemming from a landmark civil rights settlement in 2003.

Joyner himself had harsh words for the monitor in a 2020 interview with The Chronicle.

“Warshaw, for whatever reason, has treated the city of Oakland as his personal annuity,” he said. “There’s a saying, that if you’re teaching a class and five people fail, they didn’t study. If 20 people fail, you didn’t teach.”

Even as he held a high position in the Police Department, he maintained ties in his community.

Reygan Cunningham, who worked with Joyner for years, remembers marching with him through East Oakland, after the murder of 3-year-old Carlos Nava in 2011.

“I had never been out in the community with a cop where people came out of their houses to acknowledge him and say ‘What up E,’ ‘How you doing, Ersie?’ ” Cunningham recalled.

In 2013, the department tapped him to head Ceasefire, Oakland’s flagship violence prevention program that had limped along since its inception in 2007, viewed during that period as a low priority. Joyner steered it through a phase of expansion, in which the city beefed up staff and began its targeted interventions with suspected gang members. Joyner and other officers began doing “custom notifications,” in which they would visit potential recruits to the program as soon as they were released from jail, usually accompanied by a pastor.

“He was never afraid,” said Cunningham, who served as Ceasefire’s civilian program director until 2018, a position that required her to go with Joyner to visit the homes of people who had recently been shot.

“He was always like, ‘No, these are my brothers, these are my people,’ ” Cunningham said. “He would say, ‘Folks got major issues with the Oakland Police Department. ... I got issues with them, too. And I understand that because I’m a Black man who grew up in Oakland.’ ”

Oakland’s investment in Ceasefire appeared successful. A 2019 study by the Giffords Law Center to Prevent Gun Violence found the city had cut its annual number of shootings and homicides nearly in half since 2012.

That progress began to unravel after Joyner retired from the department in 2019, around the same time that other top leaders left — creating instability in the program. As the city reeled from increased violence, abetted by the pandemic and economic shutdown in 2020, Joyner shifted to a new career in the cannabis industry.

He described himself in interviews as a business owner and consultant, devoting the same fervor to cannabis that he once did to busting gangs and cracking homicide cases.

“I’m a cannabis king, now,” Joyner recently told The Chronicle. He complained about heavy taxes levied on the industry, calling it “taxation without representation.”

Days ago, Muhammad spent a night hanging out with Joyner in Dallas after an airline canceled their connecting flight from Indianapolis. They had visited the city to discuss the model of violence prevention they had employed in Oakland, which the Indianapolis mayor is now seeking to replicate.

“We talked a lot about violence in Oakland, lamenting about the work we did there together for years,” Muhammad said.

By Saturday night, Oakland police were grappling with 115 homicides — the last of which was the individual Joyner shot and killed.

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