A week-long international operation at Bulgaria's borders ended with seizures of drugs, weapons, fake goods and undocumented migrants, and investigators also tracked down the leaders of a migrant-smuggling ring they had been watching for months, said chief commissioner Anton Zlatanov, head of Bulgaria's border police directorate.

Border police, the customs agency and agencies from at least four other countries took part. They checked the main border crossings and key transport routes.

"At checkpoints we caught nine migrants hidden in specially built compartments in various vehicles," Zlatanov said. Officers also found drugs, weapons and counterfeit goods.

Sharing information with foreign partners makes it possible to reach not just the drivers but the people running the smuggling networks, Zlatanov explained. He said getting to the organizers behind these schemes is the main goal.

Customs reported several drug cases at the border with Turkey, along with seized shipments found during checks at the Danube Bridge and Sofia airport. Officers there called it a particularly worrying trend that attempts to smuggle firearms are becoming more frequent.

Over the past four months, officers have found more than 450 pistols, and one recent check alone turned up more than 1,100 magazines.

During the operation, officers also found boats being smuggled across the Bulgarian-Turkish border — boats smugglers use to carry migrants across the English Channel. To stop the supply of boats and equipment, Bulgarian authorities are working with Britain's National Crime Agency. Shipments along the route from Turkey to Western Europe have been checked for years because of the risk they carry.

Days earlier, in a separate operation, police detained 22 people from a network that guided and moved migrants through Bulgaria.

Border police continue to act against organized human-trafficking groups.