(NewsNation) — Forty years after the infamous Tylenol murders, which killed several people with laced Tylenol pills in Chicago and elsewhere and made safety seals standard practice for packing pills, the investigation is still ongoing, the Chicago Tribune reported Thursday.

A journalist for the newspaper became aware that investigators are still working on pinning the deaths on longtime suspect James W. Lewis for the potassium cyanide-laced pain killers while conducting a nine-month investigation into the murders for an upcoming investigative series and companion podcast that debuted Thursday.

While Lewis, 76, has long denied being responsible for the deaths, the Boston resident spent 12 years in federal prison on extortion charges after sending a letter to Tylenol’s parent company, Johnson & Johnson, demanding payments to “stop the killing.”

“It’s an active investigation,” one of the Chicago Tribune investigative reporters on the case, Christy Gutowski, told WGN-TV, “Investigators just got back in Illinois (Thursday) from being in Boston, the Cambridge area, and interviewed James Lewis, the prime suspect.”

While law enforcement previously lacked physical evidence directly linking Lewis to the pill bottles, the Tribune reported that investigators have come across new findings that are “chargeable” and “circumstantial” to the case.

“Lewis’ testimony to FBI reporters during a 2007-08 undercover sting operation contradicts elements of his extortion letter, as technological innovations revealed the letter has an Oct. 1, 1982, postmark hidden under layers of ink,” the Tribune reported.

This means if Lewis was telling the truth, the letter was written before the public became aware of the laced Tylenol pills.

“We were able to look at undercover FBI video that was taken in 2007 right here in Chicago at the Sheraton Hotel,” Gutowski said.

The newspaper also reviewed an old poisoning handbook that had been in Lewis’ Kansas City home before he moved to Chicago in 1981.

Provided by the University of Virginia and the National Center for the Analysis of Violent Crime, experts reportedly found Lewis’ fingerprints on several pages, including one explaining how much cyanide is needed to kill the average person.

While the evidence is circumstantial, law enforcement officials say they’re bringing the case to prosecutors in Cook and DuPage counties — where many of the deaths took place — so they can consider criminal charges.

Lewis, however, maintains his innocence, telling the Tribune in an interview last month he’s been an unfair target since the beginning of the investigation.

“Have you been harassed over something for 40 years that you didn’t have anything to do with?” he asked.

An Illinois State Police spokesperson confirmed to WMAQ-TV Thursday that they’re involved in the most recent investigative follow-up efforts, which have included meetings this summer, but “cannot comment any further because it is an ongoing investigation.”