South Korean police are investigating 17 US soldiers and five other people for allegedly using or smuggling synthetic marijuana through military mail.
This follows raids in at least two US army bases in May, including Camp Humphreys, its largest overseas base.
A Filipino and a South Korean have been arrested, while prosecutors review the cases against all 22 suspects.
A tip from the US Army’s enforcement arm had sparked a four-month investigation by Korean authorities.
According to Cha Min-seok, a senior detective in South Korea, it was one of the greatest crimes involving American soldiers in recent memory.
At the homes of the 22 suspects, 77g (2.7oz) of synthetic cannabis, more than 4kg of “mixed liquids” for vaping, and a total of $12,850 (£10,440) in cash were discovered during joint raids by South Korean police and the US Army’s Criminal Investigation Division.
They are accused of using the US military’s mail system to smuggle synthetic marijuana. It is also known as K2 and Spice, into the nation.
Seven of them, including five soldiers, are thought to have been involved in the sale of the drugs; 12 were users and three acted as middlemen. A soldier’s spouse and another soldier’s fiancée are also involved.
The 17 soldiers are currently stationed at Camp Humphreys, about 48km (30 miles) south of the capital Seoul, and at Camp Casey, an army outpost about 40km north of Seoul, according to the police.
They allegedly distributed the drugs on the bases while communicating via Snapchat.
United States Forces Korea said on Wednesday that it was aware of the investigation. No soldiers are currently in confinement or being detained in relation to it, it said in a statement.
Synthetic marijuana is made to mimic THC, the main psychoactive ingredient in marijuana.
While it has similar effects to cannabis, it is typically more potent and has been reported to produce adverse health effects, including acute psychotic episodes, paranoid delusions and severe agitation.
It is difficult to detect because it is often used in liquid form in legal e-cigarette devices, the police said.
In South Korea, those convicted of trafficking marijuana face from five years to life in prison. Drug possession carries a maximum sentence of five years in prison or a fine of about 50m won ($37,200; £30,300).