Toronto police say a rash of suspected drug-related deaths in a three-day period is highly unusual for the city, while Mayor John Tory says he is “extremely concerned” about the overdoses, which prompted investigators to issue a public safety alert on Saturday.

Four people have been found dead since Thursday, police said.

“It’s quite unusual,” said Const. Craig Brister, a police spokesman.

Overdose calls are something officers deal with all the time in Toronto, said Brister, but the number of cases since Thursday is out of the ordinary.

“What’s unique about this situation is the fact that we’re seeing such a number of them in such a close time together and within a relatively close proximity in the city, most of it has been focused in the downtown area,” he said.

Police say there have been a total of 20 overdoses in the city’s downtown core in a three-day period and police are worried about the possibility of fentanyl-laced heroin.

Investigators say the most recent death occurred Saturday when a woman was found dead in a stairwell.

Police say it appears her death was drug-related, but weren’t sure if fentanyl-laced heroin is to blame.

“Every one of these overdoses is a tragedy and each loss of a life has a devastating impact on families, friends and the community as a whole,” the mayor said in a statement released by his office on Sunday, adding the city has been developing and implementing plans to reduce drug overdoses.

“As these recent sad events demonstrate, this remains a significant challenge and we will continue to work with Toronto Public Health, the Toronto Police Service, other governments and other agencies to help people and prevent future deaths,” the statement said.

“My thoughts and prayers are with the families and friends of those we lost late last week and on this weekend.”

Police can only speculate as to the source of the drugs responsible for the overdoses at this point but want to make sure the public is warned.

“We have had a significant number of overdose incidents in the last couple of days which is alarming to us and that’s why we’re trying to put out as much information as we can to the public so there is an awareness,” Brister said on Saturday night.

Police say anyone coming into contact with a substance suspected to be heroin should exercise extreme caution and should contact police immediately if something is spotted in parks or public areas.

Many Canadian cities have grappled with drug overdose deaths in recent months, perhaps the most notable being Vancouver, which recorded 25 deaths and nearly 600 overdose calls last month alone.

More than 1,000 people died of overdoses in British Columbia last year and the coroner says fentanyl was involved in many of those deaths.