A New Jersey mayor is accused of threatening a federal agent on a public street after he mistakenly thought the agent -- who lived on his block -- was conducting surveillance on his home, days after the FBI had searched City Hall as part of a corruption investigation, according to law enforcement sources. But Union City Mayor Brian Stack denies the allegations, saying he only approached after the agent appeared to be confronting his girlfriend. Law enforcement sources first told NBC 4 New York that Stack thought two agents who had been parking their official vehicles near his home along New York Avenue were watching him as part of the corruption investigation. Stack and two others allegedly charged one of the agents in the alleged Dec. 1 confrontation and began threatening him, officials and one witness told NBC 4 New York. But the federal agent was not an FBI agent and is not involved in the ongoing Union City corruption probe.