DALLAS (AP) – Investigators are trying to determine what led a Texas teenager to fatally stab his brother and his television-personality mother before mortally wounding himself, police said Monday.

Police in the Dallas suburb of Plano said 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 at the family’s home. Authorities said he called 911 at some point and admitted to the killings.

Plano police spokesman David Tilley said a motive for the murder-suicide remains unclear, noting that there were no witnesses to the attack and that Utu didn’t leave a note. Police said they weren’t aware of any past calls from the home.

Tilley said investigators are conducting interviews with family and friends who say Utu’s disposition dramatically changed after he suffered a concussion during a high school basketball game in the fall of 2013, and then another concussion a few months later during an altercation with another student.

Utu’s uncle, Scott Fawcett, said his nephew’s mood took a dramatic turn about a year ago, and that he was receiving psychiatric care and concussion therapy.

“He had just completely mentally dwindled away,” Fawcett told Dallas TV station WFAA, where Stacy Fawcett provided weekly segments on her favorite dishes and other food-related coverage.

Utu’s father, McCann Utu Sr., said in a statement to WFAA that “God will use my pain to help and save others that are going through mental illness and concussion.”

Sarah Stoddard, an assistant professor in the University of Michigan’s School of Public Health, said an eight-year study she was involved in indicated that young people who suffered a brain injury were more likely to engage in violent behavior. Stoddard, who isn’t involved in the Utu investigation, said personality changes can result from damage to the brain’s frontal lobe.

No details have been released about the type or severity of Utu’s concussions or his medical history.

A memorial service is planned for Tuesday for the family.