Capano, 48, is charged with killing Anne Marie Fahey, the scheduling secretary to Delaware Governor Tom Carper. Fahey disappeared in June 1996.
Capano's other long-time mistress, Deborah McIntyre, 48, has told authorities that she purchased a gun for Capano just weeks before Fahey disappeared.
The sources say that Capano approached a fellow inmate at Gander Hill Prison in Wilmington, and sought his help in having McIntyre's home burglarized - to send her a message.
But one of Capano's attorneys cautioned against believing tales told out of prison.
``You're talking about an informant with a criminal history as long as my arm,'' said Capano lawyer Eugene Maurer Jr.
``He befriended Tom, who's been in prison 23 hours a day with no human contact. He obviously was very frustrated and wrote some things down and then, when he tried to get it back, [the informant] went to authorities,'' Maurer said.
Authorities allege that Capano shot and killed Fahey, 30, when she tried to end their three-year affair, then dumped her body in the Atlantic Ocean off Stone Harbor, N.J. He was arrested in November after two of his brothers provided key evidence.
About the same time that Capano approached the inmate, he was calling McIntyre from prison and trying to persuade her to stop working with prosecutors.
McIntyre had allowed her phone to be tapped and prosecutors recorded Capano's intimidating phone calls, sources said.
Maurer called the latest series of leaks a calculated effort ``to destroy my client's right to a fair trial.''
Capano, a former state prosecutor and ex-statehouse aide, has denied killing Fahey.
She disappeared after having dinner with Capano at Ristorante Panorama in Old City, Philadelphia. Her body has never been found. Capano's trial is set to begin in October, but he has asked to have it moved up to June. If convicted, he may face the death penalty.