Four employees of the National Pension Service Investment Management (NPSIM) have been under criminal investigation over suspected drug charges, police said Friday.

According to the Jeonbuk Provincial Police Agency, the four employees, who had previously served as fund managers for the world's leading pension operator, have been booked without detention on suspicion of inhaling cannabis, a banned substance in South Korea.

The four, whose identities were withheld, confessed to using cannabis during questioning, said the police agency in Jeonju, the capital of North Jeolla Province, where the NPSIM is located.

The agency said it has yet to obtain material evidence to back up their criminal charges.

The suspects' urine tests showed a negative reaction, and the National Forensic Service is now examining their hairs to check whether they took cannabis, it added.

The NPSIM is the world's third-largest pension operator with assets of 752 trillion won ($648 billion) under its management as of June. The NPSIM and its parent organization, the National Pension Service, have relocated from Seoul to Jeonju, about 250 kilometers south of the capital, as part of the government's balanced regional development policy.

The police probe began in July when the NPSIM filed a criminal complaint against the four employees after in-house rumors circulated about their alleged drug abuse. The state-run agency excluded all of the four from duty and ordered them to stand by for an appointment before reporting their cases to police.

According to police officials, the suspects allegedly inhaled cannabis at one of their homes from February and June, while one of them bought the drug via a social networking service.

The officials speculated that the suspects may have encountered the drug while studying in the United States, where cannabis is legal. (Yonhap)