Police opened fire within seconds after responding to a report of masked teens, according to a resident

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.