Four people, including a National Information Resources Service(NIRS) supervisor, are under investigation in connection with a fire at a Daejeon data center that took down hundreds of government systems.

The Daejeon Metropolitan Police Agency said Wednesday that its suspects include an NIRS site manager, two workers involved in relocating lithium-ion batteries and a worker from the project’s supervisory firm.

Investigators had questioned 12 people potentially linked to the battery relocation the previous day and placed three of them under investigation. The agency determined that the NIRS supervisor, whom it has also named a suspect, was part of on-site management but not a legally designated safety supervisor.

Police have secured CCTV footage and are conducting forensic inspections to determine the exact cause.

Six batteries believed to have ignited the blaze were sent to the National Forensic Service for analysis, one of which was found to contain residual current.