A 32-year-old Thornton man allegedly shot his neighbor in the back of the head after yelling at him for swatting his dog after the animal jumped a fence and nearly hanged itself Tuesday night, the victim’s fiancee says.

Thornton police have arrested Michael Kourosh Sadeghi for investigation of first-degree murder in connection to the death of his 42-year-old neighbor, Dustin Schmidt.

The victim has not been identified by the Adams County coroner, but Schmidt’s fiancee, Vicki Branaghan, 42, said her partner was killed while standing in their backyard in Thornton. Branaghan said she and the victim had been living together for three years.

“We were all three standing there,” Branaghan said. “Mike pulled the gun out and shot Dustin in the back of the head five times. I was just a few feet away. Mike turned around and walked back to his house as though nothing happened.”

Thornton police spokesman Matt Barnes said Wednesday that detectives continue to investigate what led up to the shooting.

“Right now no motive has been established,” Barnes said. “All indications are that these people — up until last night — got along. We’re still trying to figure out why one neighbor shot another.”

The shooting has stunned the middle class neighborhood. Terri Van Fossen said she ran outside after the shooting to comfort Branaghan, her next door neighbor and close friend. She said Sadeghi and Branaghan had been friends for a long time.

“I was so shocked last night. I can’t fathom it. They’ve always gotten along fine. There was no feud between them. Nothing,” Van Fossen said.

Branaghan confirmed she had a friendly relationship with Sadeghi, who lived alone in a two-story home that shared a back fence with the home where she lived with Schmidt.

In October, Branaghan adopted a German-shepherd mix dog, Bruno, from an animal shelter. The dog repeatedly jumped the 5-foot privacy fence into Sadeghi’s yard, she said. Branaghan and Schmidt ran electric wires on the fence and got a shock collar, but Bruno could not be stopped from leaping over the fence.

Around 7 p.m. Tuesday, Bruno was on a red leash tied to a stake in Branaghan’s grassy back yard when he jumped the fence and was left hanging by his neck on Sadeghi’s side of the fence, she said.

Schmidt ran over to the fence, reached his arms over and pulled Bruno back into his yard. He then swatted the dog. Sadeghi saw the incident from inside his home, Branaghan said.

“Mike started screaming: ‘Did you hit that dog?'” Branaghan said.

Branaghan yelled back, acknowledging that her fiance had swatted the dog for discipline, she said.

Sadeghi began yelling and cussing at Schmidt as he walked back to the house with Bruno, accusing him of beating the dog.

Schmidt then walked back to the fence where he met Sadeghi.

Branaghan said her fiance was shocked at Sadeghi’s rage over the incident and was trying to calm him down. But Sadeghi kept yelling. Schmidt had turned to walk back to his house when Sadeghi reached over the fence and shot him, Branaghan said.

Branaghan crouched on the ground and put her fiance’s head in her lap. Bruno ran to Schmidt and then around a tree again and again until he was tangled up, said Vicki’s 32-year-old brother, Will Diles.

Sadeghi didn’t speak after the shooting, Branaghan said.

Wednesday morning, blood was spattered on the fence and pooled under a tree.

“In the end of the day a man shouldn’t lose his life over it,” Diles said of the fight. “He was an awesome man.”

Branaghan said her fiance was a manager at a Longmont warehouse.

“He was loud and funny,” she said. He would eat Bruno’s food to get the dog to eat it.

After her son Skyelar Martinez fell off the back of his friend’s car and died on Dec. 18, 2015, Schmidt was the one who helped her deal with the tragedy.

“They both died here,” Branaghan said. “I need to move. I can’t stay here.”