An off-duty New York City policeman is credited with alerting Wisconsin law enforcement about online threats to kill an officer.

Officer Gregory Santora contacted police in Madison, Wisconsin, after spotting a Periscope video at home on Saturday.

In the video, a man was "implying that he was going to livestream the shooting of the police officers," Santora told New York's Daily News (http://nydn.us/2b8M5Wh ). "That's what really got me scared."

Santora called the video "eerie."

"He was crying. He was emotionally distraught," Santora said. "You could tell that he was troubled. He was not rational."

The video showed the armed man driving around, saying he was going to the place where a white officer killed a black man last year and would shoot the first officer he saw there.

About an hour later, the man approached a Madison officer parked in his squad car, who was aware of the threat, and was arrested without incident, authorities said.

Santora said he was spurred to action because he is haunted by the 2014 killings of Officers Rafael Ramos and Wenjian Liu in Brooklyn. Liu and Ramos were shot to death in their vehicle without warning by a man who had announced online that he planned to kill police officers. The suspect then killed himself.

"I didn't want to see that happen in the city of Madison," Santora said. "I wanted to see this individual be taken into custody without any violence."

Wisconsin's Dane County sheriff's office said Monday that Raynarldo Glenn was awaiting a court appearance. There was no immediate information on an attorney who could comment on his behalf.