SAO PAULO, Brazil (AFP) — A 16-year-old Brazilian boy upset over having his cell phone taken away shot and killed his parents and sister in the family’s home, police said Wednesday.

The triple homicide took place Friday in Sao Paulo, but only came to light Monday when the boy called police and confessed, the local security ministry said in a statement.

Investigation chief Roberto Afonso said the teen, who is adopted, grew “very frustrated” after his parents took his phone away following an argument.

The boy told investigators he got hold of the service gun belonging to his father, a municipal policeman, and shot him in the back.

He then went upstairs and shot his 16-year-old sister in the face. When his mother arrived home several hours later, he killed her with the same weapon, police said.

The father was 57 and the mother 50, they said.

“We need to understand if this was frustration related to some sort of psychological disturbance,” Afonso told Brazilian network TV Record.

Police are also investigating whether someone else was involved, including whether the boy spoke to anyone on the phone, he added.

The teen remained in the house with the three bodies from Friday until early Monday, when he was arrested.

During that time, he went to the gym and made a purchase at a bakery. Still “angry” on Saturday, he stabbed his mother’s cadaver with a knife, Afonso said.

The boy, who is being held at a juvenile detention center, had a “cold” demeanor when discussing the crimes, he said.

Minor defendants are protected by special legislation in Brazil and cannot be tried as adults.