Urooj Khan, 46, died suddenly as he was about to collect almost $425,000 (£264,000), but his death was initially attributed to natural causes.
The local coroner reopened his case after a relative came forward.
Chicago police confirmed they were now investigating Khan’s death as a homicide.
“It’s pretty unusual,” Cook County Medical Examiner Stephen Cina said, commenting on the rarity of cyanide poisonings. “I’ve had one, maybe two cases out of 4,500 autopsies I’ve done.”
Mr Cina’s (Free-Pr-Online.com) office found that Khan died shortly after ingesting a lethal dose of cyanide.
‘Everything to me’
Urooj Khan said he had sworn off gambling after making a trip to Saudi Arabia for the Muslim hajj pilgrimage, local store employee Ashur Oshana told the Associated Press.
Khan, who owned several dry cleaning stores, was named as the lottery winner and presented with a giant cheque in an Illinois lottery ceremony days after purchasing his winning ticket.
“Winning the lottery means everything to me,” he said on 26 June, adding that he would put some of his winnings into his business and donate money to a children’s hospital.
He opted to take his winnings in a lump sum of just over $600,000, which amounted to about $425,000 after taxes.
Khan’s winner’s cheque was issued on 19 July, the day before he died, but was cashed on 15 August.
If a lottery winner dies, the money typically goes to his or her estate, a spokesman for the Illinois lottery said.
At the time, the medical examiner’s office did not generally perform autopsies on those older than 45 unless the death was suspicious. A basic toxicology screening came back negative and Khan’s death was ruled a result of the narrowing and hardening of his arteries.
Deborah Blum, a poisons expert, said cyanide would taste strongly bitter.
The poison, Ms Blum said, disrupts the ability of cells to transport oxygen around the body, causing a convulsive, violent death. A lethal dose can kill within five minutes.
“It essentially kills you in this explosion of cell death,” she told AP. “You feel like you’re suffocating.”
The medical examiner’s office reopened the case and tested for a range of chemicals after a relative asked authorities to look into the case further.
Mr Cina refused to identify the relative.