The investigation into an ambush attack on two Los Angeles County deputies is "progressing well," with detectives now looking for someone who may have spotted the gunman, Sheriff Alex Villanueva said Thursday.

During a brief news conference, Villanueva said a man was caught on video "standing in the walkway when the shooter ran by."

"That person is a witness," the sheriff said.

The deputies were shot in the head as they sat in a marked patrol vehicle near a transit station in Compton on Sept. 12. A manhunt has failed to find the gunman.  One of the deputies was released from the hospital Wednesday, while the other remained hospitalized.

Villanueva told reporters that investigators are following "numerous and substantial leads."

During the same briefing, investigators detailed the Aug. 31 shooting death of Dijon Kizzee, who they said was armed with a loaded 9mm semiautomatic handgun and was attempting to pick it up from the ground when deputies opened fire.

Los Angeles County Sheriff Alex Villanueva comments on the investigation of the shooting of two deputies during a news conference at the Hall of Justice in downtown Los Angeles on Thursday. Investigators are looking to identify a potential witness to the attack, he said. (AP Photo/Damian Dovarganes)

An autopsy has not been completed pending toxicology reports.

Sheriff’s Capt. Kent Wegener said Kizzee was shot 19 times and died of multiple gunshot wounds, taking several bullets to the chest. Lawyers for Kizzee's family have claimed he was shot more than 20 times.

Officials said Kizzee was riding his bicycle the wrong way in traffic when a pair of deputies attempted to stop him for a code violation in the Westmont area, near south Los Angeles.

Kizzee refused, fled on his bike, and a foot chase ensued. As he was fleeing, he was holding clothing that contained a firearm, authorities said.

When deputies caught up with him, Kizzee physically struggled with a deputy and punched one in the face. Wegener said the pistol dropped to the ground and Kizzee motioned to pick it up, leading to the shooting.

He was pronounced dead at the scene. The gun was loaded with 15 rounds, Wegener said.

The weapon was reported stolen from a Las Vegas home in February 2017, Wegener said. He noted that Kizzee, 29, was barred from possessing a firearm because of his criminal record, including several felonies and a restraining order that was active at the time of his death.

The killing prompted a wave of protests amid a national debate over policing and racial injustice.

Villanueva pushed back on accusations from critics that deputies over-police certain areas of Los Angeles. He cited crime statistics in a 1-mile radius in the neighborhood where Kizzee was shot.

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"This is not your average community across America unfortunately," he said, as he projected slides detailing the number of gun crimes and assaults in the area. "So when somebody asks what we're doing in the community, why are stopping... or why are we detaining individuals, this should give you a very clear idea of what we're doing and why we do it."

"We're trying to save lives, plain and simple," he added.

At a news conference that followed, a representative for Kizzee's family called for Villanueva to resign.

“He is blaming a Black man for his own murder by the sheriff’s department," said Najee Ali, according to KTTV-TV. "And they find it incredible that they can blame the victim for his own murder. Dijon was riding his bicycle and minding his own business, and got stopped for a traffic code violation."

Calls have increased for the embattled sheriff to step down following Kizzee's death and his resistance to department oversight.

On Thursday, several members of the Civilian Oversight Commission for the sheriff's department called on him to resign, citing his fractured relationship with the county Board of Supervisors and alleged lack of transparency, the Los Angeles Times reported.

The department has made headlines recently for a number of controversial matters, including the deputy-involved shooting of an 18-year-old that was ruled a homicide, alleged deputy gangs within a number of sheriff stations and, most recently, the arrest of a local reporter covering protests outside a hospital where the two deputies wounded in Compton were being treated.