Investigators are trying to determine why a Texas teenager fatally stabbed his younger brother and mother, who was a Dallas television personality, before mortally wounding himself.

Police in the Dallas suburb of Plano say 19-year old McCann Utu Jr. attacked his 45-year-old mother, Stacy Fawcett, and his 17-year-old brother early Friday before stabbing himself multiple times.

Authorities say he called 911 at some point and admitted to the killings.

Police spokesman David Tilley said Monday that investigators are conducting interviews with family and friends who say Utu's disposition dramatically changed after suffering two concussions within a span of a few months.

A memorial service is planned for Tuesday.