A US medical examiner has said a lottery winner was fatally poisoned with cyanide a day after a cheque was issued to him for nearly $425,000.

With no signs of trauma and nothing to raise suspicions, the sudden death of Urooj Khan just as he was about to collect his lottery winnings was initially ruled a result of natural causes.

However, medical examiners, responding to a relative's pleas, did an expanded screening and determined that he died shortly after ingesting a lethal dose of cyanide.

The finding has triggered a murder investigation, the Chicago Police Department said.

"It's pretty unusual," said Cook County Medical Examiner Stephen Cina, commenting on the rarity of cyanide poisonings.

"I've had one, maybe two cases out of 4,500 autopsies I've done."

Mr Khan, who owned a number of dry cleaners, bought the ticket in June at a convenience store near his home in the West Rogers Park neighbourhood on the city's northside.

He scratched off the ticket, then jumped up and down and repeatedly shouted, "I hit a million," Mr Khan recalled days later during a ceremony in which Illinois lottery officials presented him with an oversized cheque.

He said he was so overjoyed he ran back into the shop and tipped the clerk $100.

"Winning the lottery means everything to me," he said at the 26 June ceremony, also attended by his wife, Shabana Ansari; their daughter, Jasmeen Khan; and several friends.

He said he would put some of his winnings into his businesses and donate some to a children's hospital.

Instead of the full $1m over instalments, Mr Khan opted to take his winnings in a lump sum of just over $600,000.

After taxes, the winnings amounted to about $425,000.

The cheque was issued from the state Comptroller's Office on 19 July, the day before Mr Khan died, but was cashed on 15 August, lottery spokesman Mike Lang said.

If a lottery winner dies, the money typically goes to his or her estate.

Mr Khan was pronounced dead 20 July at a hospital, but Mr Cina would not say where Mr Khan was when he fell ill, citing the ongoing investigation.

No signs of trauma were found on Mr Khan's body during an external exam and no autopsy was done.

Mr Cina said this was because the Cook County Medical Examiner's Office did not generally perform them on those age 45 and older unless the death was suspicious.

The cut-off age has since been raised to age 50.

A basic toxicology screening for opiates, cocaine and carbon monoxide came back negative, and the death was ruled a result of the narrowing and hardening of coronary arteries.

Cyanide can get into the body by being inhaled, swallowed or injected.

Use of cyanide in killings 'rare'

Deborah Blum, an expert on poisons who has written about the detectives who pioneered forensic toxicology, said the use of cyanide in killings has become rare in part because it is difficult to obtain and normally easy to detect, often leaving blue splotches on a victim's skin.

"The thing about it is that it's not one of those poisons that's tasteless," Ms Blum said.

"It has a really strong, bitter taste, so you would know you had swallowed something bad if you had swallowed cyanide.

"But if you had a high enough dose it wouldn't matter, because ... a good lethal dose will take you out in less than five minutes."

Even a small amount of fine, white cyanide powder can be deadly, she said, as it disrupts the ability of cells to transport oxygen around the body, causing a convulsive, violent death.

"It essentially kills you in this explosion of cell death," she said. "You feel like you're suffocating."

A relative came forward days after the initial cause of death was released and asked authorities to look into the case further, Mr Cina said. He refused to identify the relative.

He said: "She [the morgue worker] then reopened the case and did more expansive toxicology, including all the major drugs of use, all the common prescription drugs and also included strychnine and cyanide in there just in case something came up.

"And in fact cyanide came up in this case."

The full results came back in November.