The Federal Bureau of Investigation is examining whether convicted murderer Theodor Kaczynski - known as the Unabomber - was involved in a series of poisonings involving Tylenol painkillers in Chicago in 1982.
Kaczynski, 68, denies ever possessing potassium cyanide.
The BBC reports he was sentenced to prison for life in 1998 after killing three people and wounding 23 others in a string of bombings over 18 years.
The FBI said that a "re-examination of the evidence" had prompted the move.
Pain relief pills Tylenol were laced with cyanide, prompting a huge scare.
There was a huge recall of capsules from the shelves of pharmacies across the United States.
In a statement on Thursday, the FBI in Chicago said it had attempted to secure DNA samples from numerous individuals, including Kaczynski.
"To date, he has declined to voluntarily provide this sample. The investigation into the Tylenol murders remains ongoing. No arrests have been made and no charges have been filed," the statement said.
Kaczynski was known as the ''Unabomber'' because his early targets were universities and airlines. He was eventually turned in by his brother.