SKOWHEGAN, Maine — A discarded cigarette butt led to the arrest of Jay Mercier in a 31-year-old murder case.

Mercier, of Industry, was charged with murder on Sept. 28 in the July 5, 1980, death of Rita St. Peter and pleaded not guilty on Oct. 6. On Monday, he was denied bail by Superior Court Justice John Nivison, said Deputy Attorney General William Stokes.

Maine State Police Detective Bryant Jacques said in an affidavit that on Jan. 15, 2010, he interviewed Mercier outside his home in Industry. Mercier was smoking a cigarette and discarded it in the roadway while the two discussed the St. Peter investigation. After the meeting, Jacques collected the cigarette butt and delivered it to the Maine State Police Crime Laboratory in Augusta for processing.

Forensic DNA analyst Cathy MacMillan examined a DNA sample retrieved from St. Peter’s body and the cigarette butt obtained from Mercier, said the affidavit. She found that DNA profiles were consistent as having originated from St. Peter and Mercier.

Another DNA sample was collected from Mercier after a search warrant, Jacques said in the affidavit.

In the June 8, 2011, report, MacMillan concluded “to a reasonable degree of scientific certainty” that the sperm found on a sample taken from St. Peter’s body matched that of Mercier.

The “estimated probability of selecting an unrelated individual at random from the FBI Caucasian or the FBI African American population having a DNA profile matching Jay Mercier’s twelve locus DNA profile is less than 1 in 300 billion,” MacMillan said in the affidavit.

The affidavit also shed light into how St. Peter died. A cause of death had never been previously released.

According to the affidavit, St. Peter’s body was found about 500 feet down a field road off Campground Road in Anson.

“Photographs of the scene show Rita St. Peter positioned on her back, with significant blood on her face and head as well as the ground nearby,” said Jacques in the affidavit. “Her shirt was open, and her bra was torn, her pants and underwear were located part-way down her thighs.”

It also appeared that she had been run over by a vehicle, said the affidavit.

“Her injuries are consistent with her having been run over with a vehicle while she was prone; however, some of the injuries to her head were more likely caused by a different weapon,” said former Chief Medical Examiner Dr. Henry Ryan in the affidavit.

A set of snow-tire-type tracks was found near the body.

On July 6, 1980, Mercier consented to have investigators search his 1980 GMC pickup truck. Inked prints of all four tire impressions were taken from Mercier’s vehicle.

Forensic scientist Alicia Wilcox examined photos of the tire impressions taken at the crime scene and compared them to the known test impressions from Mercier’s vehicle, said Jacques in the affidavit.

In Wilcox’s report, dated March 25, 2006, she “noted that two of the partial impressions photographed at the scene had the same tread pattern as the outside portion of the rear tires said to have come from Mercier’s truck,” read the affidavit.

Jacques said in the affidavit that Mercier told him he never met St. Peter. Mercier said he could have gone to Campground Road on the night St. Peter was killed but doesn’t remember doing so.