GREAT FALLS, Mont. —  The man accused of plotting to kidnap David Letterman’s (search) 16-month-old son confessed after FBI agents said they wanted to give him a polygraph test, court records show.

Kelly Frank (search), 43, faces felony charges of solicitation and theft and a misdemeanor charge of obstruction. He pleaded not guilty and was being held on $650,000 bail.

Frank initially denied any knowledge of a kidnap plot when FBI agents interviewed him March 15, the documents said. But he gave investigators a signed confession when agents returned the next day with a polygraph examiner, the records show.

Frank, a painter who had been hired do to work on Letterman’s ranch in north-central Montana, was arrested after an acquaintance told investigators he had planned to kidnap the talk show host’s son, Harry Joseph, and the boy’s nanny.

The acquaintance said Frank talked about holding the two for 48 hours in the belief that he could extort $5 million from Letterman. According to court records, Frank said he had a key to Letterman’s house and even knew where the boy slept.

Frank’s attorney, Jim Hunt, has said his client “has agreed there were discussions about it, but with no purpose of carrying it out.” Hunt characterized the discussion as a “lighthearted conversation.”

The documents filed in District Court in Cascade County also said officers wanted to search for a possible ransom note on a computer and financial papers that might reveal a possible motive for the alleged plot.