A pizza deliveryman has been arrested after police say he placed a threatening phone call over a years-old grudge, leading officers to uncover a massive cache of weapons in his home.

Police say the investigation began when 32-year-old Robert Csak, of Lindenhurst, made a threatening phone call to an employee at the Summit School in Nyack. Police say Csak had previously attended the school, which serves students with emotional and behavioral issues.

Officials say Csak left a threatening message that said he felt slighted by the school employee from an encounter back in 2002.

Suffolk County Police Chief Stuart Cameron says police subsequently searched Csak’s apartment, where they found hundreds of high-capacity magazines and 19 guns, nine of which were illegal. Police say they also recovered night vision goggles, bulletproof vests, gas masks, knives, asps and a bump stock device.

“We've received a lot of threats recently this year, far more threats than we normally have. This is by far the most alarming threat that I have seen,” says Cameron.

The couple renting the apartment to Csak says he's lived there for about four years, but they never suspected anything until police showed up Saturday night.

Csak is facing a slew of charges, including multiple counts of criminal possession of a weapon. He is due to be arraigned in court on Monday.