The first 911 caller reported seeing a man walking down a street shooting a gun in the air. Authorities said they received five more similar calls from witnesses alarmed by what they saw.
Responding Los Angeles County sheriff's deputies told the man to drop the weapon, investigators said. When he refused and started toward a gas station where a family was pumping gas, they opened fire, killing the man.
Video captured the encounter Saturday in the South Los Angeles suburb of Lynwood, clearly showing Nicholas Robertson, 28, carrying a gun, crossing the street and deputies opening fire.
Investigators said in all, deputies fired 33 times at Robertson.
"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," homicide Capt. Steven Katz said Sunday in response to questions about why the deputies kept shooting the man after he fell.
Authorities released a close-up from security footage showing Robertson stretched out on the ground with a gun in his hand.
Robertson, who was black, was killed as police use of force draws increasing criticism after several deaths of black men by officers have been caught on video in California and throughout the nation.
The deputies confronted Robertson as they investigated 911 calls from the witnesses who said a man was walking down a residential street and then through a busy commercial area holding a weapon and acting strangely.
Witnesses told authorities that Robertson fired six to seven rounds and briefly went into a car wash and a pizza parlor before deputies arrived.
The deputies spotted him in front of a gas station, where two women and three children were inside a car, and ordered him to drop the gun, Katz said. Robertson refused and at one point pointed the gun in the deputies' direction, Katz added.
He said the gun was not loaded but that detectives found two live rounds "in his grasp." 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 with his spouse at home before he went out on the street, but authorities have yet to verify that report, Katz said.
Robertson's wife told the Los Angeles Times that her mother-in-law had called her shortly before the shooting to say Nicholas Robertson was under the influence of alcohol. Nekesha Robertson said she was on her way to get him and had stopped at the same gas station to buy milk before the shooting.
Nekesha Robertson said her husband was a stay-at-home father devoted to the couple's 7-year-old daughter and 6-year-old twins -- a boy and a girl -- and was not a criminal.
"Anytime you see him, you see him with the kids," she 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."
Other family members said the shooting was not justified and that Robertson may not have heard the deputies' command 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."
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.
"There's gonna be criticism anytime there's a deputy-involved shooting. We've seen that in the last two years or so, the sentiment across America has been critical," McDonnell said. "That's why we ... try to be as transparent as we can with the information that we can share."
Robertson's death came the same month that five San Francisco officers shot and killed Mario Woods, 26, after they say he refused commands to drop an 8-inch knife. Police were responding to a stabbing report Dec. 2 when they encountered Woods. Several bystanders filmed the shooting, and their videos circulated online widely.