A black man who was fatally shot by Los Angeles County Sheriff's deputies kept holding a gun as he lay dying on the ground, authorities said Sunday in response to questions about why they continued to fire on the man after he fell to the pavement.

A close-up from security footage showed 28-year-old Nicholas Robertson stretched out on the ground with a gun in his hand. He died at the scene Saturday morning in the south Los Angeles suburb of Lynwood.

Two deputies fired 33 bullets at the man after he refused to drop the gun and walked across a busy street to a filling station where a family member was pumping gas, homicide Capt. Steven Katz said.

"When he collapsed, his arms were underneath him, and the gun was still in his hand. There was never a time when the weapon was not in his possession," Katz said.

Asked if the officers were white, Katz said no but would not elaborate. He said one had been working in the field for about a year, the other for about 18 months.

Police confronted Robertson as they investigated 911 calls from witnesses who saw a man firing a gun into the air. Witnesses said he was walking down a residential street and then through a busy commercial area holding the weapon and acting strangely.

Witnesses told authorities that Robertson reportedly fired six to seven rounds and briefly went into a car wash and a pizza parlor before deputies arrived.

Deputies spotted the man in front of the gas station, where two women and three children were inside a car, and they ordered him to drop the gun, Katz said. But he refused and at one point pointed the gun in the deputies' direction, Katz added.

The gun was not registered to Robertson and has not been reported stolen. Detectives are trying to track it, Katz said.

Robertson may have been in a dispute at home with his spouse before he went out on the street, but authorities have yet to verify that report, Katz said.

Video, apparently from a cellphone, appeared on several media sites. It appears to show deputies firing about two dozen bullets, including several rounds after Robertson falls and is crawling on the ground.

"They shot him in his shoulder, and he was crawling," Pamela Brown, Robertson's mother-in-law, told Los Angeles television station KCAL. "He left three kids behind, two daughters and a son. What, they could have Tasered him or anything."

Robertson's wife declined to speak with the Associated Press, providing only a photo of their children. Earlier, she told the Los Angeles Times that her husband was a stay-at-home father who didn't engage in crime.

"Anytime you see him, you see him with the kids," Nekesha Robertson said. "He'd take them to and from school. Help them with homework. He's a daddy — that's his job. He didn't do nothing else."

Nekesha Robertson said her mother-in-law had called her shortly before the shooting to say her husband was under the influence of alcohol. She said she was on her way to get him and had stopped at the same gas station to buy milk before the shooting happened.

Other relatives said the shooting was unjustified and that Robertson may not have heard the deputies' call to drop the gun.

"This man never turned at you and looked at you or pointed the gun at you. Nothing," said Nekesha Robertson's cousin, Monica Reddix. "What they did yesterday was ... point-blank murder."

Robertson's death comes at a time of increasing criticism of police use of force after several killings of black men by officers have been caught on video in California and throughout the nation.

On Dec. 2, five San Francisco officers shot and killed Mario Woods, 26, in the city's gritty Bayview neighborhood after they say he refused commands to drop an 8-inch knife he was carrying. Police were responding to a stabbing report when they encountered Woods. The shooting was caught by several bystanders, and their videos circulated online widely.

Los Angeles County Sheriff Jim McDonnell promised the investigation into Robertson's death would be handled "with the utmost professionalism and integrity" and urged anyone with information to come forward.

"In this modern age of cellphone video and instant analysis on the Internet, I would ask that we keep in mind that a thorough and comprehensive investigation is detailed and time intensive," he said in a statement. "It will involve, not just one source of information, but numerous sources, potentially including multiple videos, physical evidence and eyewitness accounts."