Thornton, 39, who is charged with arranging the killing of his business partner, Michael DiMartino, in October 2008, became an informant in March 2008 after telling detectives he could link DiMartino to organized crime, Det. Blaise Cimilluca said Thursday at Thornton's trial in Suffolk County Court in Riverhead.
Questioned by Assistant District Attorney Nancy Clifford, Cimilluca said Thornton promised documents showing DiMartino took loans from organized crime figures, but Thornton never delivered them. He also told detectives about an upcoming meeting between DiMartino and a reputed mobster named John Costallino in August 2008, he said.
"It was my understanding it never took place," he testified, adding several other meetings also failed to materialize.
Thornton has pleaded not guilty to charges of first-degree murder and first-degree conspiracy. His attorneys say prosecutors lack evidence linking Thornton to the death of DiMartino, 44, of Nesconset.
Prosecutors believe Thornton wanted DiMartino killed because he knew about mortgage frauds operated by Thornton.
During his seven months as an informant, Thornton claimed Costallino was a brother of late mob boss Paul Castellano, Cimilluca testified.
"He said [Costallino] was like an uncle to him," he said. "He expressed fear [about being an informant] because there was an organized crime connection and he feared for his safety."
Thornton claimed to have knowledge of other mobsters, including a person named Sal Capone, Cimilluca testified. Cimilluca said he never found Costallino, Capone or any of the others.
Prosecutors have said John Costallino, or John Castallano, was a fictional person fabricated by Thornton.
On Wednesday, a detective from the district attorney's office, Det. Reinhardt Andersen, testified Thornton failed to provide incriminating evidence against DiMartino.