7NEWS found Rollin Oliver's Facebook and Twitter accounts, where he uses the nickname "Boogie" and posted photographs showing him throwing apparent gang signs.

7NEWS blurred Oliver's face in a photograph, because police are still conducting witness photo lineups in the officer's slaying investigation.

Oliver's Facebook page said he lives in Park Hill and attended Montbello High School.

Under employment, Oliver says he's been "Treasurer at park hill pharmaceutical," an apparent drug reference, since he was born in December 1990.

"Jesus is my big homie!!!!" Oliver said on Twitter, where he uses the handle @Boog_Da_Bank."

"A soldier diez once...but a coward diez 1000 timez," Oliver said in another tweet.

Denver Police Chief Robert White said officers arrested Oliver on investigation of first-degree murder. He is being held without bond.

He had a weapon on him when he was arrested, White said at a Monday morning news conference.

Oliver appeared in Denver court Monday, wearing a jump suit with his head held low.

Officer Hollis, a 32-year-old single mother, was shot in the head while trying to break up a fight Sunday night during the free weekly jazz concert in the park.

White said he doesn't believe that that gunman purposely shot the officer, and may have been firing at random or at something or someone else.

"I don't believe he purposely shot at the police officer," said White, who like fellow officers at the news conference was wearing a black band across his badge.

White thanked the witnesses who helped officers make the arrest.

"After the officer was struck, a citizen gave us a description. Shortly thereafter, several officers noticed the individual and were able to apprehend him. It is unknown whether or not he is a gang member," White said.

Video and audio recordings obtained by 7NEWS captured the panic in the concert crowd after gunfire erupted, the urgency of fellow officers radioing for paramedics to treat Hollis and the tense search for the fugitive gunman.

"Ladies and gentleman, please exit the park on the south side," a man said over a loud speaker, directing people away from the shooting.

Soon, officers were radioing that Hollis had suffered a "wound to the head."

"Get in here quick,…dammit. We've got an officer shot," an officer shouted into the radio.

"We need paramedics directed in here now," another officer radioed.

Other officers reported that the shooter was "still armed with a gun. Last seen towards the Museum (of Nature and Science)."

"We've got a guy with a gun over here by the entrance to the museum," a breathless officer radioed.

Police soon had a man in custody.

The CALL7 Investigators confirmed through an official familiar with juvenile court that Oliver was charged with a weapons offense in 2008 when he was 17 years old. He also was cited for possession of less than an ounce of marijuana in 2009, but he pleaded guilty to the lesser charge of possession of drug paraphernalia, according to court records.