The police in a small New Jersey town called a 25-year-old man into their headquarters in June after they began investigating a report that he had sent a photo of his ex-girlfriend wearing parts of a Nazi uniform to her employer, officials said.

After he arrived, the police said they discovered that the man, Michael V. Zaremski, was carrying a loaded handgun in his jacket. Concerned, officers were sent that same day to search his home, where they found a cache of assault-style rifles, ammunition and a trove of white-supremacist paraphernalia, literature and images, officials said.

Mr. Zaremski was arrested by the Franklin Borough police and eventually indicted on 39 separate criminal counts in August, according to the Sussex County prosecutor’s office.

His case, the second involving stockpiled weapons and Nazi paraphernalia in Sussex in two months, highlighted how a resurgent white supremacist threat associated with violent and deadly incidents has taken root in communities across the United States.