Police have arrested all four suspects of a blackmail scheme that involved distributing drinks laced with drugs to high school students in Seoul's Gangnam area.

According to the Seoul Metropolitan Police Agency, the last suspect, a woman in her 20s was apprehended at 11:50 p.m. Thursday in the city of Daegu on charges of violating the Narcotics Control Act.

She is accused of tricking students into drinking the beverage containing methamphetamine in the Gangnam area, known for its private academies, on Monday, by claiming that it helps improve memory.

The suspects worked in pairs near two subway stations, saying it was a tasting event and asking the students for their parents' contact information, who the group then allegedly blackmailed by threatening to report the children to the police and school for using drugs.

The suspects said they had no idea the drinks had drugs and they volunteered for the job from an online recruitment ad. Police are tracking down the alleged mastermind, suspecting the involvement of phishing scam or drug distribution rings.