The man was shown on a video posted on YouTube and was believed to have been an accomplice to the primary shooter at the Saturday event, said police spokesman Sonny Jackson. Police had urged the public to watch the video, which showed a man wearing a brown and white checkered shirt walking away from the shooting scene as sirens wail in the background.
Jackson said Monday afternoon that the man came in voluntarily and spoke to detectives. "He gave us some information, and we're moving forward with our investigation," Jackson said. "That's where we stand."
Jackson declined to elaborate further or say whether the possible accomplice was a suspect or person of interest in the case.
As was the case with the Boston Marathon bombings, in which investigators appealed for the public's help in identifying suspects captured on photos and video cameras, Denver police have asked those at the rally to share anything they may have seen, photographed or recorded at the event.
Jackson said police have received numerous tips based on videos and photographs provided by revelers at the 4/20 marijuana celebration at Civic Center Park, and he encouraged the public to keep sending any images captured around the time of the 5 p.m. shooting.
The YouTube video cited by police had been viewed more than 17,000 times by Monday afternoon.
"This day and age, we've done this a lot where we've released video," Jackson said. "It's not unusual for the public to be responsive."
Still unknown is the identity of the primary shooter. Police said they are looking for a man who was wearing a blue hat, gray sweatshirt and black pants at the rally. Denver's police gang unit is in charge of the investigation, though it wasn't clear if the shooting was gang-related, Jackson said.
Saturday's shooting occurred at the end of an outdoor celebration of the first 4/20 counterculture holiday since Colorado legalized marijuana last fall. The jovial scene turned to panic when the shots were heard and thousands fled the park.
A man and a woman suffered non-life threatening gunshot wounds. A juvenile grazed by a bullet was treated at a nearby hospital.
It wasn't clear if the shooting would affect future plans by organizers of the annual rally, which traditionally attracts tens of thousands of pot smokers to downtown Denver. Festival organizers did cancel a Sunday event after the shooting.
Rally organizer Miguel Lopez did not return phone calls and emails Monday seeking comment.
Last summer, a Denver police officer was killed in what police called a gang-related shooting that disrupted a crowded jazz concert in Denver's City Park.
Rollin Oliver, 22, pleaded guilty to second-degree murder and faces at least 16 years in prison when he is sentenced at a June 21 hearing. Oliver was arrested after witnesses provided descriptions to police.
Officer Celena Hollis, 32, was killed while responding to a disturbance at the concert. Oliver's attorneys say he was being pursued by gang members when he drew his weapon and fired.