Police say they’ve caught the criminals responsible for a string of violent grocery store holdups last year in Calgary and Airdrie.

After a five-month investigation dubbed Operation VOLCOM, police arrested four men and laid 15 robbery-related charges.

The thieves, who were armed and wearing disguises, typically arrived as the stores were closing.

They would round up the employees, order them to the ground and force one of them to open the safe.

During a robbery of a grocery store in Airdrie, the culprits kidnapped the manager to gain access to the safe. She was later released unharmed.

Two of the men who were arrested have already pleaded guilty in connection with the robberies and are now serving prison sentences of three and 7.7 years, police said.

During the investigation police learned that similar robberies were being planned at 10 more businesses, leading to 50 conspiracy-related charges against another six men.