SACRAMENTO -- The NBA's Sacramento Kings and Boston Celtics have taken up the cause of a young black man killed in a police shooting. Before Sunday night's game, the teams wore t-shirts bearing Stephon Clark's name. Officers fired 20 shots at Clark. They say they thought he had a gun, but they only found a cellphone.

"We must say loud and clear that what happened to Stephon Clark was plain wrong," Sacramento Mayor Darrell Steinberg said. "It's wrong, because a 22-year-old man should not die in that way."

Clark's grandmother stood with national civil rights leaders on Monday and described the moment he died in a hail of police gunfire.

"All that I heard was boom, boom, boom, boom, boom," said Sequita Thompson.

Those shots were captured on police body cam video. Clark was in his grandmother's backyard when he was killed.

SACRAMENTO, CA - MARCH 26: Sequita Thompson, (C) grandmother of Stephon Clark who was shot and killed by Sacramento police, cries as she speaks during a news conference with civil rights attorney Ben Crump (R) on March 26, 2018 in Sacramento, California. Justin Sullivan / Getty Images

"They didn't have to kill him like that," Thompson said. "They didn't have to shoot him that many times."

Civil rights attorney Ben Crump joined the family's call for justice.

"This is reminiscent of so many police shootings of unarmed black and brown people," Crump said.

Clark had been spotted by a sheriff's helicopter as a suspect breaking car windows. Two patrol officers pursued him and within seconds of reaching him opened fire. They said they thought he had a gun.

"So here's the common element between the community and the officers is fear, it is fear," said Steinberg, who has seen his city rocked by protests. "It is rather ironic that the city's aggressive video-release policy in some ways has not lowered the temperature. It has raised it, because the video itself raises those questions."

One question raised by the video is why did officers turn off the sound recording on their body cameras minutes after the shooting as they examined Clark's body. Investigators will want to know what they were talking about.