The massive data breach that compromised the personal information of more than 30 million Coupang customers appears to have been caused by an insider.

According to industry sources on Sunday, a former Coupang employee is suspected of leaking the data.

But there are concerns that the investigation may face challenges, as the employee in question is a Chinese national who has already left South Korea.

Coupang said Sunday that it cannot confirm the details but is strengthening monitoring to ensure that no further information is leaked.

The e-commerce giant announced the previous day that personal information from about 33-point-seven million customer accounts was exposed.

The police received a complaint about the situation from Coupang on Tuesday last week and have since launched an investigation.

The complaint submitted by Coupang did not identify a suspect, referring to the offender only as an “unknown person,” but the e-commerce company previously indicated that the leak was not caused by a hacker or other external factors.