DENVER -- A Denver man was shot multiple times and killed during a medical marijuana transaction gone awry, police said.

The coroner identified the victim as 59-year-old Douglass Singleton.

Neighbors said Singleton began cultivating medical marijuana in his house on the 2600 block of Leyden Street 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.”

Singleton’s son told 7NEWS that he was at his father’s house when the shooting happened.

“You have no idea what that was like,” said an emotional Justin Singleton.

When asked if the suspect was a regular 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.”

Neighbors didn’t learn about the shooting until police went door-to-door.

“I heard someone knocking on my door at 1:30 in the morning,” Latta said. “I wasn’t about to answer it, so I called 911. The dispatcher told me it was police. They told me there had been a shooting next door. I thought it must be Doug.”

A short time later, a man matching the description of the shooter walked into a Denver hospital with a gunshot wound. Police aren’t divulging how the suspect was shot, or who shot him.

It appears the two men were conducting a medical marijuana transaction when they got into a fight, according to police. They believe the suspect pulled out a gun and shot Singleton.

The injured suspect is being treated for his wounds at the hospital.

Neighbors told 7NEWS that Singleton was a veteran of the Vietnam War and that he had grown frail and was using bottled oxygen.

The shooting occurred in a house just two doors up from the Montessori Children’s House of Denver.

Officials there declined comment about the case.

Medical marijuana advocates said that caregivers can grow a limited amount of medical marijuana in their homes as long as a patient registered with the state has listed them as their caregiver.

Brian Vicente of Sensible Colorado told 7NEWS, that caregivers don’t have to notify anyone, but he said medical marijuana cannot be grown in a way that endangers the public nor in view of the public.

Between July and mid-December, there were 25 medical marijuana-related robberies in Denver, police said. Those crimes include 13 dispensary sticks-ups and 12 medical-marijuana-related robberies at residences, where targets ranged from pot growing operations to home invasion robberies of medical marijuana patients.