(CNN) -- South Korea said on Wednesday it is indicting a U.S. Army private for his involvement in the biggest drug smuggling case in the country.

The soldier, from the 2nd Infantry Division based in South Korea, smuggled in about $90,000 worth -- or 3,400 grams -- of synthetic marijuana, also known as Spice, through international mail from Hungary and the United States since last year, an official from Seoul Central Prosecutor's office said. The official declined to be named because it is an ongoing case.

The private will be transferred to South Korean custody on Monday, according to the 2nd Infantry Division of U.S. Forces in South Korea.

"Since January 2012, the 2nd Infantry Division has cooperated fully with the Korean National Police investigation and the soldier has repeatedly been made available for questioning by the KNP and for court hearings," the division said in a press release.

"The 2nd ID will work hard to continue reducing Soldier misconduct as we stand together with our ROK allies to strengthen the Alliance and defend the peninsula," it said.

At least five more U.S. citizens, including a former U.S. soldier, will be prosecuted in the case, and authorities said there may be more people involved in the drug ring.