A woman in her 40s was arrested for offering high school students drug-infused drinks claiming that they would increase their ability to concentrate, sparking a wider police investigation.

On Monday, the suspect, with three more accomplices, had approached students on the street in Seoul's Daechi-dong, Gangnam-gu, the country's premier district for private education, feigning a promotional event for a new drink. Contained in a bottle with a sign that reads, "Mega ADHD," the free drink was distributed to students under the promotion that the "newly invented drink will help boost memory and strengthen concentration," according to the police.

Philopon, or methamphetamine, and ecstasy were detected during a reagent test the police conducted on the victims. The police are investigating a total of six cases related to the incident as of Wednesday.

Meanwhile, the police are accelerating the investigation considering the possibility of a mastermind behind the crime, ordering the suspects to provide drug-infused drinks to many unspecified individuals.

The expansion of probe comes after the suspect claimed she had not known the drinks contained drug substances. Apprehended at around 1:30 a.m. at her house in the neighborhood of Dongdaemun, the suspect is reported to have said she was just applying for a part-time job position she had found online.

Analyzing the CCTV footage, the police identified a total of four people separated into two groups had worked under a systemized structure. A man in his 40s, whom the police was after, turned himself in at 10:00 a.m. after a slew of local media reports on the incident.

The police are still after two more accomplices, women in their 40s and 20s respectively.

Officials allege the suspects have also blackmailed the parents of the students who have consumed the drink. After the teens took the drink, the culprits had asked for their parents' number to "check purchasing decisions" and threatened they would report to police the students have abused drugs.

The number of the victims could rise as there are witnesses that say such testing events had happened frequently near the crime site in the last few months . Some students have also reported that unidentified people have handed out 'concentration boost' drinks in front of their schools and have seen similar drink being advertised on social media.

The suspects face charge of violating the Narcotics Control Act. Recently, a revision to the Narcotics Control Act was proposed to the National Assembly to require those who administered or provided drugs against the will of others to be sentenced to more than three years in prison. Compulsory administration against the will of others requires stricter punishment, but under the current law of South Korea, there is no provision for aggravated punishment.