The gun used to kill a Duluth man could not have discharged from accidentally being dropped on the ground, authorities allege in a new criminal complaint.

Brian Ross Shaw, 35, denied pulling the trigger in the December 2018 incident that killed 35-year-old Kevin John Weiss in the Gary-New Duluth neighborhood. He reportedly told investigators he had the safety on when he stumbled backward and dropped the 12-gauge shotgun, which fired when its butt struck the ground.

But prosecutors now say that account is impossible. Testing conducted by the Minnesota Bureau of Criminal Apprehension allegedly showed that blows to any part of the pump-action firearm would not cause it to discharge, regardless of whether the safety was activated.

"Only manually removing the safety lever to the 'fire' position and pulling the trigger will discharge the weapon," authorities said in the amended complaint.

Sixth Judicial District Judge Jill Eichenwald on Thursday approved a prosecutor's request to add a charge of intentional second-degree murder against Shaw, who was already facing a lesser charge of second-degree manslaughter.

With the ruling, a jury trial that had been scheduled for Jan. 7 will be postponed. Shaw will instead appear in court that day to raise any constitutional or probable cause challenges to the new charge.

Authorities said Shaw shot Weiss during a Dec. 10 confrontation after days of expressing "extreme hostility" toward the victim. Shaw reportedly has a terminal illness and had spent several months stockpiling firearms before the shooting, according to court documents.

Weiss was pronounced dead after suffering a gunshot wound to his lower abdomen during the incident outside a residence on the 100 block of West Reis Street. Shaw allegedly claimed to have shot him by accident, though authorities have disputed that account.

According to the complaint, Shaw had lived with a woman at the residence for a number of years and had three children with her. After they separated in May 2018, Shaw moved to Hibbing. Weiss, who grew up with Shaw, then began a relationship with the woman, but she had kicked him out of the house on the afternoon of the shooting.

Weiss later returned to collect his belongings and broke a window in the home, according to the woman, who had said she wanted no contact with him. The complaint states that she was in contact with Shaw, who acknowledged to police that he had also talked to Weiss and told him he had a shotgun. But he denied making threats.

Shaw allegedly told investigators that he brought an uncased shotgun with him to the residence. He said that when he arrived, he encountered Weiss outside, and that Weiss threw a container at him. The defendant stated that Weiss yelled at him and eventually pushed him, causing him to stumble backward and fall, accidentally discharging the gun.

After shooting Weiss, Shaw allegedly entered the house and told the woman, "I shot the f---er." In a recording of a 911 call, he could be heard telling the woman to make sure to inform the dispatcher that it was an act of self-defense, according to the complaint.

In addition to the firearm testing, authorities said a preliminary autopsy report found a downward trajectory of the four shotgun pellets in Weiss' body, inconsistent with Shaw's account.

St. Louis County prosecutor Nate Stumme filed a motion to add the murder charge on Nov. 22, saying the new BCA testing provided enough evidence for a jury to conclude that Shaw acted with intent to kill Weiss.

Defense attorney Matthew Benfield objected, arguing at a hearing this week that the complaint was being amended in a "punitive and retaliatory fashion" because Shaw was exercising his right to go to trial.

Taking a few days to consider the motion, Eichenwald said a delay in resolution of the case does not infringe on Shaw's rights. He remains out of custody, having previously posted a $75,000 bond, and has not demanded a speedy trial.

"While the case is relatively old, the state's explanation for the delay in charging is reasonable given the complexity of the case and the need for forensic analysis of evidence, which takes time," the judge wrote in an order. "It was appropriate for the state not to charge the more serious offense until it had probable cause to do so."

Under Minnesota sentencing guidelines, a first-time offender convicted of second-degree manslaughter by culpable negligence faces a prison term of about four years. For intentional second-degree murder, it's in excess of 20 years.