A Lindenhurst pizza deliveryman accused of having an arsenal of illegal weapons and making threatening calls over a years-old grudge appeared in court on Monday.

Police say 32-year-old Robert Csak was arrested Saturday after he left threatening voicemails for a school security guard in Upper Nyack, New York. Police say Csak had attended the Summit School, which serves students with emotional issues, in 2002.

When police went to Csak's Lindenhurst home, they say they discovered an enormous cache of high-powered weapons and ammunition, plus other gear. Police say they found 19 long guns, nine of which are illegal in New York state. They also found a bump stock device, high-capacity magazines, a gas mask, a bulletproof vest and hundreds of rounds of ammunition.

Csak worked at Albert's Pizza in West Babylon as a driver. His boss says Csak was a quiet man, but adds that he recently noticed a change.

"Last couple weeks, he looked so angry," Csak’s boss says.

Prosecutors say they're glad Csak was arrested because they say he fits a profile that is often seen with potential active shooters.

"The FBI calls that a ‘grievance collector,’" says Suffolk District Attorney Tim Sini. "Someone who harbors animosity against a particular person, harbors that feeling, lets it build inside and then at one point makes a threat."

Prosecutors say they're not sure where Csak purchased his guns. They say federal authorities are running traces on all of his weapons.

During Csak's arraignment Monday, he was ordered not to have any contact and to stay away from the security guard or the Summit School. At that point, Csak blurted out in the courtroom, "I didn't intend to... "

The judge cut Csak off and reminded him if to his right to remain silent, and to only have his attorney speak for him in court. Csak's attorney declined to comment.

Csak was ordered held on $750,000 bond.