Denver police spent Sunday scanning video taken at Saturday’s 4/20 rally where three people were shot, and by late afternoon released a YouTube video showing a possible suspect and asked for the public’s help in locating him.

The suspect, who is walking in a crowd moving away from the scene of the shooting as sirens wail in the background, is seen about six seconds into the video, said police spokesman Sonny Jackson. “We are seeking this individual as we believe he assisted the primary suspect after the shooting. He is the black male with a brown-and-white checkered shirt (walking) between the male in the red shirt and the male wearing the white hat,” Jackson said in an e-mail.

The investigation has been turned over to the police gang unit, though it is not clear if gangs were involved, Jackson said. “They have their ears to the ground and take care of this sort of thing,” Jackson said.

Alexander Pringle, who witnessed the shooting, thought he was seeing the start of a fight when people began running near the spot where he was photographing the action at Saturday’s rally.

“Then this guy came up and started shooting at people,” he said Sunday.

The shooting turned a peaceful day of celebration for marijuana enthusiasts into a mad scramble as three people were struck by bullets.

Denver police said two people received leg wounds and were taken to a hospital with non-life-threatening injuries. A juvenile who was grazed by a bullet walked to a nearby hospital.

Pringle, 28, a photojournalism student at Metropolitan State University of Denver, was showing those standing near him a picture he had just taken of a “guy with this massive joint” when shooting erupted.

A former U.S. Army medic, Pringle watched as the shooter turned and started running.

Pringle chased him, realized there were people hurt, and came back to help.

A man was on the ground bleeding. Pringle and another man removed the victim’s pants and located a hole on the inner side of his leg.

Using a cloth belt as a tourniquet, Pringle said he tried to stop the bleeding, but the belt snapped.

“I readjusted it and tied it down,” he said.

The wounded man’s dog was also hit and sat nearby whimpering. “The dog was really calm and peaceful, especially for being shot.”

Police are asking anyone who witnessed the shootings at Civic Center to come forward.

Police are looking for two African-American men, one who was dressed in a gray hoodie and light blue baseball cap, the other who was wearing a checked shirt.

“When gunfire happens, people run away. Now that things have calmed down, we would like them to come back and talk to us,” Jackson said Sunday.

Andrew Lindsley, who was nearby when the shots rang out, said when he saw two men “coming aggressively through the crowd,” he thought trouble was on the way.

He continued walking past the two, and a few seconds later he heard gunshots and started running, he said. “It was absolutely gang-related,” he said.

Saturday’s rally — an annual event called 4/20 that is a holiday for marijuana enthusiasts — was the first since Colorado voters legalized the use of marijuana for people over 21.

Witnesses said as many as 10 shots sounded about 5 p.m. near the Greek theater, where musician Lil’ Flip was performing.

Anyone with information about the shooting should call Crime Stoppers at 720-913-STOP. Information leading to an arrest and charging of a wanted individual can bring a cash reward up to $2,000.

Tom McGhee: 303-954-1671, tmcghee@denverpost.com or twitter.com/dpmcghee