DENVER -- A simple sale of two "baggies" of medical marijuana turned into a shootout between three gunmen that left a 59-year-old Vietnam veteran dead and an accused robber wounded, police records revealed Thursday.

Marijuana has long been considered a mellow, harmless drug.

But the fatal gunfight after midnight Tuesday has alarmed residents in the Park Hill neighborhood and underscored that Colorado's growing, unregulated medical marijuana business can be a magnet for violent crime.

Peter Wilson Bellar, 26, was charged Thursday with first-degree murder and aggravated robbery in the shooting death of Douglass Singleton, Denver District Attorney Mitch Morrissey's Office said.

Bellar has a long criminal history in Colorado, including convictions for felony drug possession and misdemeanor assault in 2007 and for felony burglary and misdemeanor assault in 2006, according to court records.

Neighbors described Singleton as frail Vietnam vet in poor health who used bottled oxygen. They said he began cultivating medical marijuana in his house about a year ago.

"He mentioned that he was going to start selling medicinal marijuana, he mentioned that to someone I know," said next-door neighbor Sadie Latta. "We never saw any problems."

Things went wrong when Singleton's 30-year-old son, Justin, brought two men to his father's home at 2660 Leyden St. early Tuesday to buy pot from his dad, who the son described as a "licensed medical marijuana provider," according to a police statement supporting Bellar's arrest.

See the full arrest for Peter Bellar

While Singleton might have been a state certified medical marijuana patient, there is no such thing as a license medical pot provider under Amendment 20, the initiative Colorado voters approved in 2000 to allow patients to legally use the drug with a doctor's authorization.

Justin Singleton said one of the two men was named "Wilson" whom he'd known for years.

The son told police he gave "Wilson two baggies of marijuana that he stated he wanted to purchase," according to the arrest warrant statement. But the son said Wilson then "pulled out a black colored handgun and stated that he needed to take all the marijuana."

Justin Singleton said he went to a bedroom to tell his father that Wilson was trying to rob them.

"Justin stated that he and his father obtained two handguns from the bedroom (and) … confronted Wilson at which time shots were exchanged, and the victim was shot by Wilson," the police statement said.

Arriving officers found the elder Singleton had been shot in the head. He was pronounced dead at the scene. The coroner said he was killed by multiple gunshot wounds.

Soon afterward, police said, Bellar walked into Denver Health Medical Center, suffering from a gunshot wound to his torso. He is recovering and is scheduled to appear in Denver court Monday to be formally advised of the charges.

Justin Singleton told TheDenverChannel.com he was stunned after witnessing his father's death.

"You have no idea what that was like," he said emotionally.

When asked if the suspect was a regular medical marijuana customer of his father’s, Justin said, “No. He was an acquaintance of mine.

"I thought he was a friend, but he came over with a gun," the son said.

Park Hill residents upset by the killing and a new medical marijuana dispensary expected to soon open on East 28th Avenue were schedule to hold a community meeting Thursday.

"We are fed up and can't believe these facilities can go into our neighborhoods just feet away from homes and schools," a resident e-mailed TheDenverChannel.com.

From July 1 to the end of the year, there have been 28 medical marijuana-related robberies in Denver, police said. Those crimes include 15 dispensary sticks-ups and 13 medical-marijuana-related robberies or burglaries at residences, where targets ranged from pot grow operations to home invasion robberies of medical marijuana patients.