A US soldier and two women were detained for allegedly distributing and taking synthetic cannabinoids, according to local reports Thursday quoting police.

According to the Pyeongtaek Police Station, a Korean national is accused of taking and bringing synthetic cannabinoids into Korea via the military postal service through her boyfriend, who is a US soldier stationed at Camp Humphreys in Pyeongtaek, Gyeonggi Province. She is also accused of finding potential buyers and distributing the drugs with a Filipino woman.

In principle, all mail that comes through the military postal service must be scanned through an X-ray machine, though not all mail can be monitored due to a lack of personnel in customs clearance.

Twenty additional US soldiers were also arrested for taking and distributing the drugs in and around Camp Humphreys. A US soldier who worked as a supplier in the group is reportedly being detained in a US military facility.

“It is complicated as it involves customs and the US military,” said a police official. “We aren’t able to confirm further details on the case as it is still an ongoing investigation.”