TOKYO — Tokyo Customs punished three of its officers today for secretly slipping drugs into travelers' luggage more than 160 times at Japan's main international airport to train drug-sniffing dogs, an official said.

The three customs officers had been planting drugs in randomly selected bags at Narita International Airport in Tokyo since September last year, Tokyo Customs spokesman Kazutoshi Takahashi said.

"We are deeply sorry that such acts have happened," Takahashi said. "The three officers apologized and explained that they did it in an effort to boost the dogs' performance."

The trio's monthslong practice was exposed following an embarrassing blunder in May.

One of the officers slipped a package of cannabis resin into an outside pocket of luggage belonging to a traveler from Hong Kong, but a sniffer dog failed to detect it, Takahashi said.

Once the officer realized the drugs had left the airport — along with the unsuspecting traveler — he panicked and informed his bosses.

Tokyo Customs then frantically sought help from airline and airport officials to track down the Hong Kong traveler at his Tokyo hotel and recover the more than 4 ounces of resin a day later.

"This embarrassing incident prompted us to investigate," Takahashi said. He insisted that the three had never unlocked or unzipped passengers' bags.

"They only placed drugs in (outside) pockets of luggage," he said.

Takahashi did not say where the officers had obtained the drugs in the first place.

The officer who lost the drugs in May is to be suspended from duty for three months, while two others will have their salary cut by 10 percent for three months.

The head of Tokyo Customs was also given a verbal warning for failing to oversee the operations, and eight other senior officers were either warned or given temporary pay cuts.