From an old article I found:
(subscription to Newsbank required to view the whole story)
Wisconsin State Journal (Madison, WI)
August 20, 1991
Author: Denise Sanders Police reporter
KIDNAPPER, VICTIM, ONCE IN MADISON
``There were about three calls placed from Madison,'' said special agent Kent Miller of the Madison FBI office.
Miller said it was not known whether the two were headed away from Madison or were returning to the area. The last call was placed from a phone booth May 9, 1986.
Miller said a letter postmarked from Madison in June 1986 was the last known written communication. Miller confirmed that three letters were mailed locally to the girl's parents, Nancy and Kim Marshall.
``He was traveling through, either a salesman or a truck driver - something that resulted in his living on the road,'' Miller speculated.
According to the producers [of Unsolved Mysteries], the kidnapper wrote extensively to the parents about how he and the girl, now 12, had traveled around the country and even to Europe. He told them he was raising Nyleen as his own child and had no intention of returing her to her parents.