Mandell allegedly planned to kidnap, torture and murder Steve Campbell in 2012 as part of a plan to steal money and real estate holdings. Federal prosecutors said Mandell planned to use a fake arrest warrant to pick up Campbell.
Campbell testified on Wednesday. He said a handwritten letter was left at his home with a business card from Steven Mandell. He told the jury that he spoke with a man listed on the number on the card.
"What was it about the conversation that caused you to go to police?" Assistant US Attorney Diane MacArthur said.
"The commercial properties, those dots don't connect to my primary residence," Campbell said. "The amount of errors in the statements sufficiently raised my radar . . . It says to me that someone was looking deeper than the surface and got on my radar."
Campbell said he was managing rental properties in 2012. Prosecutors said Mandell was going after Campbell's cash and rental property when he came up with the plan.
Mandell's attorneys said the ex-cop was just talking on tapes recorded by the FBI, and never planned to act out any of his plans. Mandell is expected to take the stand in his own defense.
Mandell's alleged accomplice, Gary Engel, committed suicide after his arrest.