Thieves have targeted Canada Post workers in Montreal in a series of attacks on their letter carriers.

The criminals would steal the postal worker's keys to access apartment buildings and mailboxes throughout the city.

Montreal police said they are aware of about a dozen cases since December 2014 in the northeastern parts in Montreal, with one theft in Laval.

Police Sgt. Raphael Bergeron said the thieves are most likely trying to steal welfare cheques. They have not received too many complaints for missing letters or packages.

Bergeron said the investigators could not confirm if all the thefts are linked, because “some may be copycat crimes.”

Union spokesperson Alain Duguay told CJAD 800 News that in most cases postal workers were attacked by two suspects. He also confirmed that one employee was robbed twice in a matter of days.

"Some are very nervous, we're telling our members not to be confrontational, give the keys and collaborate with police," he said.

No postal employees have been seriously injured, said Duguay.

He said a string of robberies like these has happened before.

"We went through this three years ago and it was organized crime. We just hope the police find the thieves soon because it's worrying our members."

Canada Post spokesperson Anick Losier said they are extremely concerned for the safety of their employees and for the security of the mail.

“We have had a rash of incidents in the Montreal-North, St-Michel area,” confirmed Losier. “We’re working closely with police to be able to catch the criminals and to make sure our employees are safe.”

Losier wants to assure the public that when criminals steal keys from the letter carriers, all the locks are immediately changed.