A teenager who tried to return a malfunctioning replica gun that shoots plastic projectiles to a sporting goods store in Washington state was shot to death by an off-duty security guard who believed the boy was holding a real gun and planned to rob the business, according to authorities.

The man arrested in the killing, 51-year-old Aaron Myers, told police that he was not working at the time he shot and killed Hazrat Ali Rohani, who was returning a malfunctioning airsoft gun to the Big 5 store in Renton. But he had offered to keep watch due to alleged rising crime in the area.

Investigators said security footage, which has not been publicly released, contradicted Myers’ statements to police, leading to his arrest. Authorities also said Myers is not a law enforcement officer and therefore “has not been trained in how to safely prevent crime” despite his employment as a security guard.

Rohani’s death is the latest in a spate of shootings across the US of people approaching homes or businesses for an innocent reason.

Experts say the US, which has more guns than residents, has a disproportionate number of those shootings because people assume – often incorrectly – they can use firearms with impunity thanks to lax weapons laws and self-defense statutes that are generally permissive.

Myers was allegedly waiting for his son to come out of jiu-jitsu class at the martial arts studio next to Big 5 when three teenagers walked past his car, including one who held an airsoft gun that Myers said appeared to be a Glock pistol.

Police said Myers then pointed the gun issued to him for his security job at the teens and told them to put their hands in the air. Meeghan Black, a Renton police spokesperson, told CBS that the teenagers immediately complied and tried to explain their gun was not real.

But Black said Myers fired several shots after claiming “the confrontation intensified … rapidly”, hitting and killing 17-year-old Rohani.

Police said Myers stood over Rohani and continued to shoot, which he claimed he did because he believed one of the teen’s companions was reaching for a gun in Rohani’s waistband.

A statement from police said King county sheriff’s office deputies were completing a training exercise nearby when they heard the shooting. They tried to provide first aid to Rohani, but he died at the scene, the sheriff’s office said.

The other teens present at the incident told police they were “dealing with functioning issues on the airsoft gun and wanted to take it back” to the store to either return or exchange it, officials said. The teens said they needed help with “a magazine issue [Rohani] was having with the firearm”.

In court documents, King county prosecutor Leesa Manion alleged Myers “took it upon himself to conduct ‘overwatch’ in a Renton parking lot, despite the fact that he is not a member of law enforcement and thus has not been trained in how to safely prevent crime”.

“Rather than calling 911 or waiting for any evidence at all that could confirm or deny his assumption, he claimed he had a ‘duty to intervene’ and did so,” Manion added.

Myers faces two felony counts of second-degree murder and assault. He could received a maximum punishment of life imprisonment if convicted as charged.