The family of Stephon Clark, the unarmed black father from Sacramento killed by police in his grandparents’ backyard, has filed a wrongful death lawsuit, alleging two California officers “acted with deliberate indifference” and denied the 22-year-old medical care.

The Sacramento police officers displayed “reckless disregard for the rights and safety” of the young father when they gunned him down in March 2018, the family alleges in a federal lawsuit filed Monday that names the city of Sacramento and the two officers involved in the shooting.

The policemen, Terrence Mercadal and Jared Robinet, had no information suggesting Clark was armed and had not witnessed him committing any crimes, the complaint argued. They gave no verbal warning that they would use deadly force, shot Clark after he already hit the ground, and then subsequently “prevented responding medical personnel on-scene” from providing care to Clark in a timely manner as he was “bleeding profusely”, the complaint said.

“The involved officers are responsible for every single shot they fired and this was not an immediate defense of life situation,” the complaint said.

The attorneys representing Clark’s children, parents and grandparents, further accused the police department of broad failures, including “inadequately supervising, training, controlling, assigning, and disciplining” officers, and maintaining policies and practices that allow for “excessive force” and unjustified uses of “deadly force”.

Stephon Clark and his two sons. Photograph: Courtesy of the family

The attorneys have asked for damages of at least $20m.

“The Sacramento police executed Stephon Clark in his grandparents’ backyard, mistaking his cell phone for a gun, assuming he was a criminal threat simply because he was a black man,” Ben Crump, one of the attorneys for the family, said in a statement on Monday, adding that police “ignored him for six minutes, failing to render any help to save his life”.

Susana Alcala Wood, Sacramento’s city attorney, declined to comment on the lawsuit, and police spokespeople did not respond to an inquiry. Mercadal and Robinet could not be reached.

Clark, who left behind two young sons, was shot in a confrontation with two police officers on 18 March 2018. He was holding only an iPhone when officers killed him. Body-camera videos of the shocking killing went viral last March, sparking massive protests and national outrage. The videos revealed that the officers did not announce that they were police when they arrived in the dark, shouted at Clark to show his hands, and then quickly fired a round of bullets before he could respond. The body-camera footage recorded the men shouting “gun!” and showed that they began shooting less than 20 seconds after they arrived. Both officers muted their microphones in the immediate aftermath of the shooting.

The officers were responding to calls about someone breaking into cars when they located Clark, police said at the time.

Police in America disproportionately kill young black men and boys, and Clark’s death quickly drew comparisons to high-profile cases, including the killings of Tamir Rice, Eric Garner, Philando Castile and Walter Scott.

Clark’s family and civil rights groups have long called for the two officers to face criminal charges, and the district attorney’s office said Monday that its investigation was still ongoing. The state attorney general’s office is also conducting a review. US prosecutors rarely bring criminal cases against officers who kill civilians.

“This has taken a tremendous toll on them,” Dale Galipo, another attorney for the family, told the Guardian. “They still don’t have any form of justice. There’s been no criminal prosecution. Most likely, there will not be, because there almost never is.”

Clark’s sons, who were ages one and three when he died, “miss their father much”, he said.

Galipo, who has brought other police brutality cases against Sacramento law enforcement, said the facts of Clark’s killing made for a strong case, adding: “I would hope police officers are able to distinguish a gun from a cell phone.”

Clark’s death, which inspired messages of outrage from athletes, celebrities and politicians, led Sacramento police to adopt several reforms, including a new policy meant to discourage risky foot pursuits and a rule requiring officers to verbalize a reason when turning off their body-camera microphones.

But activists dispute the reforms have had significant effect. Tanya Faison, founder of Sacramento’s Black Lives Matter chapter, said the new rules were not having an impact. That’s why criminal consequences are necessary, she said.

“Once they see they can lose their job and pension and can go to court and be treated like every other citizen … then police officers might stop shooting us,” Faison said, adding that there have been numerous other tragic killings by Sacramento law enforcement since Clark’s death, and that she hoped they could also get attention. “Those other families need justice as well.”