Laval Police are asking residents to be cautious and on guard after multiple homes received “suspicious” letters Monday.

According to a new release sent out by officials, the envelopes contain an “unknown white substance,” paired with a threatening extortion letter.

The Crimes Against the Person squad is investigating the incidents.

Laval Police are urging the public not to open the letters and to contact police immediately.

“If you have received such a letter in the last few days or notice the presence of a suspicious envelope at your home, please immediately contact your police department, via 911, to notify them of the situation,” the release reads.

The unknown substance is being analyzed.