South Korean police said Tuesday an investigation into illegal drug use in Wonju has expanded to include four soldiers and two South Korean citizens.

Police last week questioned two Camp Long-based soldiers suspected of illegally using the animal tranquilizer ketamine with two South Korean civilians at an off-base apartment.

They also questioned one of the civilians, a woman married to one of the soldiers investigated Friday.

The other Korean civilian, a man identified only as Cho, 24, is being held in police custody.

During the questioning, a police spokesman said Tuesday on the customary condition of anonymity, police learned about two more soldiers who allegedly used the drug.

The spokesman said police hope to question those two soldiers early next week.

The Korean woman is not being held. As the wife of a soldier, she falls under the status of forces agreement that defines how U.S. military community personnel are treated within the Korean legal system, the policeman said.

The policeman said everyone questioned to date has admitted to using the drug.

Police forwarded Cho’s case to local prosecutors, who now must decide whether to indict him.

The policeman said Tuesday that they plan to forward charges against the first two soldiers and the South Korean woman after they finish the U.S. military side of the case.

Eighth Army officials have confirmed that at least two U.S. soldiers are under investigation and that they remain "present for duty" with their units.

The Army is cooperating with the South Korean investigation, 8th Army officials said.

Ketamine has become popular at dance clubs and rave parties in the United States. It also is reportedly used as a date-rape drug, according to the U.S. Drug Enforcement Administration.