To his astonishment, the scratch-off ticket was a $1 million winner. But the day after the state issued the check last July, Khan suddenly died, leaving authorities with a baffling mystery and a homicide investigation.

After initially ruling that he died of natural causes, the Cook County Medical Examiner’s office now has determined that Khan, 46, ingested a lethal dose of cyanide. The case was reopened after a relative pleaded for an expanded screening, and the Chicago police now are cooperating in an investigation into who might have killed him.

“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.”

Khan, who owned a number of dry cleaners, stopped in at the convenience store near his home in the West Rogers Park neighborhood on the city’s North Side in June and bought a ticket for an instant lottery game.

Convenience store clerk Ashur Oshana told The Associated Press on Monday that Khan had gone on the pilgrimage and told him he was done gambling. But Khan couldn’t resist and scratched off the winner in front of Oshana.

“Right away he grabbed my hand,” Oshana said. “He kissed my hand and kissed my head and gave me $100. He was really happy.”

At an Illinois Lottery ceremony days later, Khan recalled that he jumped up and down in the store and repeatedly shouted, “I hit a million!”

“Winning the lottery means everything to me,” he said at the June 26 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.

Khan opted for a lump sum of slightly more than $600,000. After taxes, the winnings amounted to about $425,000, said lottery spokesman Mike Lang. The check was issued from the state Comptroller’s Office on July 19, the day before Khan died. It was cashed Aug. 15, Lang said, explaining that if a lottery winner dies, the money typically goes to his or her estate.

Calls to Khan’s family went unanswered Monday. A knock on the door at the family’s small, two-story house late Monday afternoon wasn’t answered.

Khan was pronounced dead July 20 at a hospital, but Cina would not say where Khan was when he fell ill, citing the ongoing investigation. The external exam showed no signs of trauma on Khan’s body.

No autopsy was done because, at the time, the Medical Examiner’s Office didn’t generally perform them on people 45 and older unless the death was suspicious, Cina said. The cutoff age has since been raised to age 50. After the basic toxicology screening for opiates, cocaine and carbon monoxide came back negative, the death was ruled a result of the narrowing and hardening of coronary arteries.

Cyanide can be inhaled, swallowed or injected. Deborah Blum, an expert on poisons who has written about the detectives who pioneered forensic toxicology, said using cyanide to kill someone has become rare partially because it’s difficult to obtain and easy to detect — often leaving blue splotches on a victim’s skin.

“It has a really strong, bitter taste, so you would know you had swallowed something bad if you had swallowed cyanide,” Blum said. “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.”

It takes only a small amount of fine cyanide powder to 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.”

After the initial cause of death was released, a relative of Khan’s asked authorities to look into the case further, Cina said. He would not identify the relative. The full results came back in November.

“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 I believe strychnine and cyanide in there just in case something came up,” Cina said. “And in fact cyanide came up in this case.”

Chicago Police Department spokeswoman Melissa Stratton confirmed the department was now investigating the death, and said detectives are working closely with the Medical Examiner’s Office.

Investigators will likely exhume the body, Cina said.

Oshana said he was shocked to hear that someone might have killed Khan.

“I’m very sorry for him,” Oshana said.