The police have refuted criticism that the investigation into drug use by G-Dragon of K-pop group BigBang was insufficient after the decision was made not to refer the case to the prosecution.

Kim Hee-jung, head of the Incheon Metropolitan Police that probed the case, said on Thursday that investigators at the start had received a detailed tip regarding alleged drug use by the star, whose real name is Kwon Ji-young, but did not find sufficient evidence to proceed.

The chief said that an investigation is imperative when a detailed tip is received, while also acknowledging that the police have a responsibility to close a weak case that cannot hold up in court.

The decision not to indict comes after the 35-year-old tested negative for illegal drugs in both preliminary testing and the National Forensic Service's thorough analysis.

As for actor Lee Sun-kyun, who also faces drug charges, the police plan to first investigate allegations of blackmail by a 29-year-old employee of an adult entertainment establishment in Gangnam and others against the 48-year-old actor.

The police chief said getting to the bottom of the blackmail case filed by Lee himself would help investigators verify the credibility of drug-related testimonies.

Out of ten people connected to the case that the police probed both internally and officially over alleged drug use, three, including the employee, have so far been transferred to the prosecution.