The parents of a Montana teenager who shot and killed a friend knocking on his bedroom window late at night say he should not face criminal charges over what they describe as a tragic accident.

In an exclusive interview, they told the Associated Press that they blame themselves for allowing the teen to keep a loaded revolver in his room.

As a prosecutor moves to put the case before a special jury to decide on charges, the May shooting has revived the debate over the so-called Castle Doctrine that sometimes allows the use of lethal force to defend one’s home. It has also raised questions about the 17-year-old shooter’s easy access to the handgun, given his age.

Fifteen-year-old MacKeon “Mackey” Schulte was killed as he and another boy tried to wake up Seth Culver at about 2.30am, less than a block from where the three attended high school in Billings, Montana’s largest city.

Startled by the noises at his window, Seth grabbed a second world war-era, .38-caliber handgun given to him as a present by his father and shot once through the glass, striking MacKeon in the head, according to authorities and Culver’s parents.

The case will soon place the Culver family in an unwanted spotlight: Yellowstone county attorney Scott Twito told the AP he will convene a coroner’s jury next month to recommend whether criminal charges are warranted.

MacKeon Schulte, 15, of Billings, Montana, was shot dead late at night by his close friend, 17-year-old Seth Culver. Photograph: Regina Culver/AP

In interviews this week, Len and Regina Culver said they now realise they should not have allowed their son to keep the loaded revolver in his bedroom.

Len Culver, 60, said he had instructed his son on how to handle weapons responsibly, enrolling him in a hunter safety course when the boy was 13. Culver said he did not know until police told him on the morning of the shooting that it is generally illegal under federal law for a person under 18 to possess a handgun.

“I don’t want them to make some BS, liberal deal out of this,” Len Culver said. “He did it. Yes, he did it. But let’s treat it like what it is. It isn’t going to happen again. Mack’s gone.”

The Culvers said criminal charges would merely compound a tragedy that has left their son despondent over his role in the death of one of his closest friends.

Regardless of whether the killing was intentional, police and outside experts have said Seth could face charges if it is determined that he disregarded the risks of not identifying his target before pulling the trigger.

Twito said he took the rare step of requesting the jury because of “the dynamics involved in this case”.

“The ages, circumstances, timing, relationships of the people, all of those things,” Twito said. “I think it was an appropriate case to look at in this way.” Witnesses could include the families of the two friends.

Such proceedings are open to the public and are usually reserved for fatal shootings by police officers or the death of inmates in custody. The jury’s findings would be advisory only and a final decision will be up to Twito’s office.

Twito said it was unlikely that Seth’s parents would face charges but added that he was “going to let the process unfold”.

Seth Culver, 17, is said to be despondent after shooting his friend dead. Photograph: Regina Culver/AP

Immediately after the shooting, MacKeon’s family and friends rallied around Seth Culver to show support.

MacKeon’s father, Sean-Paul Schulte, hugged Seth at MacKeon’s funeral. Others spoke of the close bond the shooter and victim shared since they met during their freshman year at Billings senior high school.

Regina Culver still carries on her cellphone a text she said she received from Sean-Paul Schulte directed to her son: “Don’t throw your life away … Mackey loved you … be strong as u can, study and do pushups,” a portion of the message read.

MacKeon’s family has not said publicly whether they want charges filed in the case. But Sean-Paul Schulte said he believes both boys had been “desensitized” to violence by the video games they played.

“It’s hard to say it’s an accident when you grab a gun and shoot someone on purpose,” Sean-Paul Schulte said. “I don’t want some other dad in my position.”

Several weeks before the killing, Seth Culver took the handgun without his parents’ permission to a sleepover at MacKeon’s house, his parents said. When they discovered the weapon was missing from the drawer by their son’s bed, he had to come home. Len Culver said his son was yelled at for taking the gun and told he couldn’t take it out of the house again without telling them.

Looking back on that incident as she stood outside the window where MacKeon was killed, Regina Culver said she wished they had gone further.

She said the gun “should have been taken away [but] we probably just took TV away from him

“We’re not the only ones that knew he had it,” she added. “Mack knew he had it.”