Pasco County Sheriff Chris Nocco said the man who shot and killed his mother's friend Thursday afternoon was suffering from mental illness.

Pasco Sheriff: Brian Disario shot and killed his mother's friend David Armstrong, 56

Disario said Armstrong had been hitting on his girlfriend, who does not exist

Disario engaged in shootout with deputies in Pasco neighborhood

Disario, who was later found dead inside home, had imaginary girlfriend, authorities said

The incident started about 3:30 p.m. when deputies closed part of State Road 52 near Meadow Drive in Port Richey due to reports of an active shooter in the Gulf Highlands subdivision. Disario had been in the garage of his home with his mother, her boyfriend and their friend David Armstrong.

Nocco said before the suspect, 30-year-old Brian Disario, barricaded himself inside his home, he believed his mother's friend, Armstrong, 56, was trying to hit on Disario's imaginary girlfriend.

"Please note Brian does not have a girlfriend. In his mind he did but in reality he didn't," Nocco said. "At one point Brian comes out and says to the victim, 'you're messing with my girlfriend.' The suspect then pulls out a long gun and shoots the victim."

Authorities said Disario then walked out into the street armed with that weapon and engaged in a shootout with deputies. All of this unfolded Thursday afternoon less than two blocks from Gulf Highlands Elementary School.

"As they're saying 'sheriff's office' he raises the gun at the deputies and starts firing rounds at the deputies in the middle of the street in a close proximity to the school," Nocco said. "Our deputies returned fire. There is a gun battle out in that street."

Gulf Highlands Elementary School was placed on lockdown, but authorities stressed that students were safe and that the shooting was in a nearby neighborhood.

Still, residents and parents scrambled for information during the chaotic scene.

"Actually, I came out and a police officer yelled at me that there was an active shooter and that we had to get back into our homes," said resident Nickie Thewes. "As we were in the house, we could hear gunshots."

After the gun battle, Disario went back into the home and barricaded himself inside. Deputies later found him dead inside the home.

It's not yet known if Disario died from being struck by a deputy's bullet or from a self-inflicted wound. Disario did not have a criminal record but officials did say multiple weapons were found inside of the home.

Neighbor Lori Petit said Disario moved into the home earlier this year. In that time, she'd witnessed domestic incidents with the family.

"[Disario] would be arguing all the time with people," Petit said. "There were constant fights, bickering back and forth, cursing, just loud. I mean, top of their lungs."

Nocco said the deputies who fired at Disario have been placed on routine administrative leave.

The incident remains under investigation.