PASCO CO., Fla. (WCMH) – Authorities in Florida say a murder-turned-shootout started when a man believed a family friend was hitting on his imaginary girlfriend.

According to the Pasco County Sheriff’s Office, Brian Desario, 30, shot and killed a friend of his mother’s, 56-year-old David Armstrong on Thursday afternoon. Desario believed that Armstrong was hitting on his girlfriend, a woman law enforcement officials said doesn’t exist.

When deputies arrived at the scene, Disario turned his gun on them, prompting a shootout and causing a nearby elementary school to go on lockdown.

According to WKRN’s sister station WFLA, deputies pulled back and attempted to negotiate with Disario before using a robot to enter the home. When deputies were able to safely enter, they found Disario dead inside. The exact cause of death is still under investigation.

Disario reportedly had a history of mental illness. Investigators said Disario managed to stockpile weapons and ammunition, but they aren’t sure how.