A California teen who told police in May his younger brother fatally shot his father before turning the gun on himself was arrested Friday, charged with killing both victims, PEOPLE confirms.According to a statement from Fresno Police, 18-year-old Robert Cotter told police he was in the family’s home on May 17 when his 15-year-old brother, Brian Cotter, shot and killed his 53-year-old father, Patrick Cotter.

Police were called to the home and discovered Patrick dead from a “gunshot wound to the upper body,” and Brian alive, but severely wounded.

Brian was rushed to the hospital, where he clung to life for several days before succumbing to his injuries.

But as the investigation progressed, detectives determined Robert’s account of the purported murder-suicide was “inconsistent with the physical evidence obtained during the investigation.”

Investigators confronted him, and the teen allegedly “made admissions, to detectives, about planning and acting out the brutal murders.”

Cotter is being held without bail.

Police have not discussed a motive.

Cotter is charged with two counts of murder. He has yet to enter pleas to the charges.

It was unclear Monday if he was represented by an attorney.