POLICE have a secret wit­ness who may prove that disgraced ex-cop DREW PETERSON murdered his missing 4th wife STACY.

The witness, a wannabe cop who washed out of the police academy in Chicago, claims he stalked Stacy at Drew’s request – and cops be­lieve he helped to dispose of her body.

The ENQUIRER is keep­ing the witness’ name secret but learned au­thorities found him through Drew’s phone records from the weeks be­fore Stacy’s disappearance on Oct. 28, 2007. They also found records of a gun transfer between them.

“Police brought (the witness) in for question­ing,” said a source. “He arrogantly went in and told investigators he didn’t have any problem talking.”

He said that he’d known Drew, a black belt in karate, for years after first meeting him in a martial arts class.

The witness also admitted he’d been fol­lowing Stacy, Drew’s fourth wife, who was telling her husband she wanted a divorce.

“He said Drew was suspicious that Stacy was cheating on him and wanted her followed,” said an insider.

But the wit­ness’ face turned red when cops asked him about a blue barrel that they say Drew carried from his home in Boling­brook, Ill., the day Stacy went miss­ing. Drew’s stepbrother Thomas Morphey had told authorities he helped Drew carry the heavy barrel from the house and put it in his SUV. Morphey believes Stacy’s body was inside.

“Once they started ques­tioning the witness about the blue barrel he flipped out,” the source said. “He immediately insisted that he needed a lawyer.”

The questioning ses­sion ended, but sources say police believe that the witness helped Drew get rid of the barrel.

Apparently spooked, the witness suddenly moved more than a thousand miles away in 2009, soon after Drew was arrested for the murder of his third wife Kathleen Savio, whose naked body was found in the bathtub of her home on March 1, 2004. The witness’ wife then sold their Chicago home and joined him.

He moved on a whim,” the source said. “Why leave your home­town? He just bailed. Guilty people act like that.”

Drew, a former police sergeant in Bolingbrook whose boorish be­havior made for national headlines, was convicted of Savio’s murder on Sept. 6. He faces up to 60 years in prison.

Will County State’s Attorney James Glasgow now wants justice for Stacy.

The ENQUIRER learned that he’ll be presenting a case for a murder indictment against Drew to a grand jury – and the secret witness is on the list of those who’ll testify.

“Glasgow may have to offer him immunity,” said the insider. “He thinks the witness has the proof that will nail Drew for Stacy’s murder.”