The two handguns found in the shooter’s possession belonged to his grandfather, while the rifle belonged to his parents, the source told French press agency AFP.
In addition to these weapons, the teenager was also found in possession of a grenade.
French police have taken a second person into custody over the shooting at the Alexis de Tocqueville high school that left 14 people injured, according to the source, who said that a brother of a friend of the shooter was being held on Friday.
The youth is being held for attempted murder.
Three students and a teacher suffered minor gunshot wounds, while the other 10 were treated for shock or light injuries incurred during the stampede triggered by the shooting. There were no fatalities.
“The statements given by students and teachers yesterday reveal that they are in total shock,” said Education Minister Najat Vallaud-Belkacem on Friday.
The minister cited a teacher, who told her that the shooter was relatively discreet at school. “I never would have imagined for one second that he could be, as we have discovered, obsessed with weapons,” the teacher said.
Vallaud-Belkacem also said that the shooter may have targeted certain students, and that he “did not act blindly, but rather had a specific objective”.
She went on to praise the teacher who had rushed towards the teenager as he pulled out his weapons to “try to reason with him”. The teacher put himself between the shooter and the student he was targeting, and was grazed by a bullet while doing so.
Authorities have said the 16-year-old was on bad terms with some of his fellow students, and that he was interested in guns.
Local official Fabienne Atzori said on Thursday that the shooter had been motivated by the “bad relationships he had with certain classmates.”
“It seems that he had trouble fitting in” at school, she added.
He had also shared images on social media of school shootings in the United States, such as the one at Columbine High School in Colorado in 1999 that left 15 people dead, including the two shooters who afterwards committed suicide.