The man, Roddie Bryan, shared the video in 36 seconds with police and cooperated with investigators, attorney Kevin Gough said in a statement Friday.
"It was Mr. Bryan who filmed the incident, revealed the existence of the videotape and invited a responding Glynn County police officer to sit with him in his truck where they watched the video together," Gough said.
The video appears to show the confrontation between Arbery and two men prior to the shooting that left Arbery dead in Brunswick, Georgia.
The men – father and son Gregory McMichael (64) and Travis McMichael (34) – were arrested Thursday and facing charges of murder and aggravated assault in Arbery's murder Feb. 23, according to the Georgia Bureau of Investigation.
The footage was "very important evidence," GBI director Vic Reynolds said Friday. Asked if there will be more arrests, Reynolds told reporters that the investigation is still active.
"We are investigating everyone involved in the case, including the person who shot the video," he said.
Authorities' comments have put Bryan's life at risk, along with his family, friends and neighbors, his lawyer said in a statement. He added that his client had lost his job, despite not committing any crime and cooperating with the investigation.
Two men arrested in Arbery shooting
McMichael allegedly chased Arbery in a pickup and thought he was a suspect in a series of burglaries, according to a police report.
Gregory McMichael told police that his son shot Arbery after the latter allegedly attacked him as he stood in the way with a shotgun, according to the police report.
According to an April 7 letter from Waycross Judicial Circuit District Attorney George Barnhill, who has since dismissed the case, the son and McMichael helped with a previous indictment for Arbery when they both worked for the Brunswick Judicial Circuit District Attorney's Office . Barnhill's letter, sent to Georgia Attorney General Chris Carr's office, did not specify what the former prosecutor's office was for or when it occurred.
Brunswick News has reported that Arbery was charged with allegedly breaking a gun to a high school basketball game in 2013 when he was 19. Family lawyer Lee Merritt has admitted Arbery's arrest in 2018 on charges of shoplifting. But any reference to "alleged behavior from high school or shoplifting is absurd and has nothing to do with his murder."
"I just want people to know that he was a very good young man and that he loved the people, and I just want people to remember him as a kind-hearted young man," his father, Marcus Arbery Sr., said Friday what would have been the son's 26th birthday. "Seeing him just being lynched like that by a racial mob is devastating for our family."
Three shots heard in piggy video
Gregory and Travis McMichael were arrested two days after Bryan's video surfaced online, prompting calls for justice in the southern Georgia community.
The video was released by a local radio personality on Tuesday. CNN has not verified who recorded the video – taken by someone in a vehicle pulling up behind a pickup that stopped in the road – but it captured incidents that match many stories about the shooting.
The footage begins with Arbery jogging down the middle of a street toward a pickup that stopped in the road. Gregory McMichael is in the bed of the truck while his son stands near the driver's door.
Arbery approaches and he and Travis McMichael begin to mute.
A shot goes off and the two disappear from the left side of the screen when Arbery appears to throw a punch. Gregory McMichael takes out a gun, but does not fire.
A new shot is heard as the men struggle outside the camera. When they return to sight, both are still throwing themselves with the shotgun. Arbery looks to throw a right-handed hit to Travis McMichael's head when the third is heard.
Blood appears on Arbery's T-shirt under the left rib. He stumbles and falls in the middle of the two-lane road.
The arbery case originally landed on the desk of Brunswick Judicial Circuit District Attorney Jackie Johnson, who resigned because Gregory McMichael had worked for the office. The case then went to Barnhill.
GBI took over the case on Tuesday evening at the request of Tom Durden of the Atlantic Judicial Circuit, which is the third and current prosecuting authority.
Politicians, activists and celebrities have weighed in on the incident in recent days, including Georgia Governor Brian Kemp, who called the footage "absolutely awful" and said Georgians "deserve answers."
CNN's Dakin Andone and Jason Morris contributed to this report.