They knew it was the wrong dog when he refused to roll over

MEXICO CITY (Reuters) - A top police sniffer dog working for an elite Mexican drug squad was stolen during an airport transfer by thieves who left a mixed-breed puppy in its place, the attorney general's office said.

Rex IV, a highly trained Belgian Malinois sheepdog with a string of drug hauls behind him, was checked on to a flight from Mexico City this week with seven other police dogs bound for an operation in the northern state of Sinaloa.

But when the dogs arrived at Mazatlan airport, Sinaloa, their police handlers discovered a small black mongrel puppy inside Rex IV's cage, with the sniffer dog nowhere to be seen.

"In 17 years I've never seen anything like this. It's rather delicate," a Public Security Ministry spokesman told Reuters on Sunday, adding that the worry was the dog could help smugglers find new ways to conceal drugs.

"It's like kidnapping an intelligence agent," he said.