The head of Quebec’s police watchdog said it will investigate the death of a 15-year-old boy who was shot dead by officers, revealing that the only weapon recovered from the scene of the fatal shooting belonged to the police.

At a rare news conference on Tuesday, Brigitte Bishop, director of the province’s office of independent investigations, said her unit would conduct an “impartial” investigation into the death of Nooran Rezayi.

On Sunday, police received a call reporting a group of masked teens who were alleged to be armed in Longueuil, a city less than 10 miles (16km) east of Montreal.

Officers arrived just before 3pm and, according to the police watchdog, “made contact with the members of the group”. Eyewitnesses say the police shouted “stop” twice before shooting towards the teens.

Resident Johnny Inthisone told CBC News that the shooting unfolded only seconds after police warned the youths to stop.

Despite efforts by police and paramedics to revive Rezayi, he was pronounced dead at a hospital.

Bishop said Rezayi was struck by at least one bullet and that investigators were conducting a forensic analysis of the gun collected at the scene.

Rezayi’s family, who immigrated to Quebec from Afghanistan, say the death of their son has left them with grief and few answers.

“I love my son. I love my baby,” his mother, Fahima, told the Montreal Gazette. “Why did they shoot my baby?”

Rezayi said the family has received almost no information since he was killed. “Nothing. They’re not giving me anything,” she said.

The teen’s father, Sharif, said police didn’t come to the family home until nearly six hours after the shooting. “We sat here, not knowing. And then they came and told us he was gone,” he said.

During the Tuesday press conference, Bishop said investigators retrieved a baseball bat and ski masks at the scene but declined to provide further specifics about the case, citing the ongoing investigation. She said five people had been assigned to the case.

But she warned residents against pursing “private investigations” into the shooting.

“A lot of people that are trying to understand what’s going on. I totally understand that, but they’re doing certain private investigations and they’re meeting with people,” she said, calling on anyone with evidence to come forward. “You have to trust us. We’re the ones who are doing the investigation. We’re an independent office. Our mission is to shed light on what happened and that’s what we’re going to do.”