Details are emerging at a sentencing hearing for a 13-year-old boy who killed his 16-year-old brother in Dorval in January.

The boy pleaded guilty earlier this month to negligence causing death with a firearm, while a charge of manslaughter was dropped.

Seeking to show the killing was not an accident, and that no one else was involved, as the boys’ mother previously claimed. The prosecution hopes to use the aggravating factors to argue for a maximum sentence, which is three years in a youth detention centre.

On the stand Tuesday was the first police officer on the scene the night of the drama. He described seeing the 16-year-old covered in blood, lying in his bed.

The officer met the then 12-year-old suspect, whom he described as badly shaken and panicked.

As the officer described the scene, the boys’ father had to step out because he was sobbing and couldn’t stand to listen to the physical description.

The next witness was ballistics expert Guillaume Arnet, who showed the court the small 9mm Beretta gun that was used in the killing. The gun is prohibited because of its short barrel.

It was seized from the boy's home along with 70 bullets. The boy also had two pellet guns.

The ballistics expert explained that the gun had a fail-safe mechanism, and it required a fair amount of force to pull the trigger. Arnet also revealed that by his own estimates, the boy was no more than 40 centimetres from his brother when he shot him in the head.

Later this week, the defence will introduce its own witnesses to argue for a lesser sentence.

It has already asked for what is called a pre-sentencing report, usually written by a psychologist or a social worker to get a clearer picture of how dangerous the boy is.

The defence is convinced the child is not a threat to anyone and plans to ask for bail while awaiting his sentence.