The police have launched a formal investigation into SK Telecom’s handling of a recent hacking incident.

The Seoul Namdaemun Police Station announced on Tuesday that it will question representatives of the Daeryun Law Firm over their complaint against SK Telecom CEO Ryu Young-sang and the company’s chief of security, which alleged negligence and obstruction of duty.

Daeryun claims SK Telecom failed to properly manage user data and accused Ryu and others of dereliction of duty and interfering with official procedures.

Police are also looking into a separate complaint filed by a civic group, which accuses SK Group Chairman Chey Tae-won of violating data protection laws by allegedly delaying disclosure of the breach.

On Monday, a joint public-private investigation team confirmed that SIM card information for all 25 million SK Telecom subscribers was compromised in a cyberattack last month.