An AI gun detection system installed in a Nashville, Tennessee high school couldn’t spot the gun a 17-year-old used to fatally shoot another student and himself earlier this week, according to district officials.

Authorities are examining “very concerning online writings and social media posts” connected to the shooter who killed a female student and wounded another student in a Nashville high school cafeteria.

The investigation into why a Nashville high school student fatally shot a classmate before killing himself has centered on his online writings, which authorities describe as concerning warning signs.