Footage released by the Sheriff's Department showed Nicholas Robertson, 28, walking down a road in Lynwood on Saturday morning carrying a gun in his left hand.
Witnesses reported seeing Robertson behaving erratically and shouting expletives, Los Angeles County Sheriff's Capt. Steve Katz said.
Katz said at one point, Robertson walked into a pizza store carrying the gun and left.
At least six businesses or residents called 911 to report the gunman.
"Those 911 calls depict different behaviors of the suspect," Katz said. "Some indicate that he was armed. ... And then there were others that indicated not only was he armed, but had actually discharged the firearm several times."
One caller said the man fired six to seven rounds as he was walking through a residential area, Katz said. He said evidence indicates that Robertson did fire his .45-caliber semiautomatic handgun in the area.
Deputies caught up with Robertson as he was crossing Long Beach Boulevard toward an Arco gas station.
"He did not comply with their repeated requests to drop the weapon," Katz said.
He said Robertson turned his body with the gun toward the deputies, who opened fire and killed Robertson.
"We do not have any evidence that indicates a round was fired at deputies at this time," the captain said.
He said one deputy fired 17 shots, and another fired 16.
"It's not necessarily unusual, and the circumstances have to be evaluated," Katz said.
He showed reporters a photo that showed Robertson holding the gun after he was on the ground.
The Sheriff's Department said every fatal deputy-involved shooting gets independent investigations from the local district attorney's office and the county coroner.
Katz said investigators are asking for the public's help in providing any additional information or videos of the incident.