A birthday party for a 5-year-old at a Commerce City home was interrupted this weekend when the family dog jumped a fence and a passer-by fatally shot the dog.

The incident, which “horrified” children as well as adult guests at the party, happened about 5 p.m. Saturday in the 14800 block of East 116th Drive, according to the family whose dog was killed.

“We heard a noise. It sounded like a birthday balloon popping, except much louder,” said Emily Martinez, the owner of the dog that was killed.

People inside the home looked out the window and saw a man with a dog on a leash, standing over the downed dog, named Clifford, and pointing a gun at the animal.

“This guy had his gun pointed down at Clifford,” Martinez said. “He shot him twice in the neck.”

People rushed from the home to help Clifford and confront the armed man. As she bent beside her dog, the man continued to brandish the gun, Martinez said, pointing the weapon at her as well as others, including children.

“I was screaming at him to leave,” Martinez said. “He was pointing the gun at people.”

Raymond Martinez, Emily’s husband, ran back into their home to grab his gun. By the time he re-emerged from the home, after a couple of minutes, the man who fired the shots had left.

Commerce City police quickly responded to the home Saturday after neighbors and guests had called 911. On Monday, the department said it is investigating the incident, adding that the shooter, who was out walking his border collie at the time of the shooting, is being cooperative.

Police said the owner of the border collie told them he shot Clifford after he was unable to separate the dogs.

Afterward, neighbors and party guests told the Martinez family the shooter was calling out, “I have a concealed weapon license,” as he walked away.

Clifford was a 4-year-old chocolate Labrador/collie mix. He was adopted from a shelter about two years ago and hadn’t had any problems or previous incidents, Emily Martinez said.

The man who shot Clifford had walked his dog by the Martinez home the past two months, typically about 5 p.m., Raymond Martinez said.

When he had first started walking by, the man had stopped, Martinez said, to complain about Clifford’s barking.

He was also concerned that Clifford might jump the fence, a split rail with mesh wire attached, Raymond Martinez said. Martinez added that he was apologetic at the time.

“There was no big argument or anything,” Martinez said of the earlier encounter. “I said: ‘Sorry about that, Clifford is not aggressive.’ ”

The deadly Saturday encounter could have been avoided, Martinez said. “In my mind, he could have walked somewhere else,” he said.

Emily and Raymond Martinez wrapped Clifford in a blanket and rushed him to a nearby veterinarian, where the dog died.

Kieran Nicholson: 303-954-1822, knicholson@denverpost.com or twitter.com/kierannicholson