The longtime suspect in the cyanide poisonings of Tylenol tablets that killed seven people in the United States four decades ago has died at age 76, police in Massachusetts said.

The case of the unsolved 1982 murders sent Americans into a panic and resulted in a nationwide safety overhaul that saw pharmaceutical companies apply protective foil seals to over-the-counter medication packaging. 

James Lewis was found unresponsive at his home Sunday and was pronounced dead shortly afterwards, police said in a statement late Monday. 

"Following an investigation, Lewis' death was determined to be not suspicious," the Cambridge Police Department said.

No one was ever charged in the killings.

But Lewis was sentenced to 12 years in jail for extortion, accused of sending a letter to manufacturers Johnson & Johnson demanding $1 million to "stop the killing."

Lewis, an accountant who left prison in 1995, has always denied orchestrating the murders. 

But according to the Chicago Tribune, he described the curse of being known as "Tylenol Man."

The widely used extra-strength Tylenol headache and fever capsules were laced with cyanide and then replaced in their packaging, which went back on the shelves in Chicago area pharmacies.

Seven people, including a child and three members of the same family, died after swallowing just one pill each.

The murders rocked the United States and stores across the nation quickly removed Tylenol from their shelves.

The investigation involved more than 100 officers, some 400 suspects and led to a 20,000-page report even though there was no crime scene and no motive.

Since then, all medicines have been packaged in hermetically-sealed containers, with warnings urging people not to use any medicines in packaging which has been tampered with.

In 2009 the FBI and Illinois state police reopened the investigation, in part due to what they said were "many recent advances in forensic technology."

Lewis was ultimately not charged in the murders, but he had a long history with the law, and he remained under scrutiny.

In 1978 he was charged with the murder of Raymond West, a Kansas City man who had hired Lewis as an accountant, according to media reports at the time. Lewis was arrested after allegedly forging one of West's checks for $5,000.

But the charges were dropped after a judge ruled that police acted improperly in arresting Lewis and searching his home.

mlm/cl