Auxiliary power may not have been shut down correctly as a fire ripped through a government data center in South Korea last month, police believe.

As reported by DCD, the battery fire at the National Information Resources Service (NIRS) data center, located in the city of Daejeon, on September 26, has caused havoc in Korea, with the majority of government digital services still offline two weeks later.

The Daejeon Metropolitan Police Agency is investigating how the fire started, and last week revealed four arrests had been made in connection with possible professional negligence.

According to a police statement reported by the Korea JoongAng Daily, officers have now questioned 26 people, arresting a fifth person.

They have also raided NIRS and three of its partner companies, impounding materials, project plans, and battery log data.

Witness testimony provided to the police suggests that, though the main power to the building was disconnected when the fire broke out, an auxiliary power system connected to the data center’s Uninterruptible Power Supply (UPS) battery backup system was not.

The fire itself started during work to move lithium-ion UPS batteries from a data hall on the fifth floor to the building’s basement. This involved disconnecting power cables.

Main power to the data center was shut off at 7.09pm, an hour before the fire broke out, but multiple witnesses reported that the auxiliary power remained connected, and this may be why the battery that initially caught fire was between 80-90 percent charged at the time, police said. Most manufacturers recommend a low or moderate level of charge before attempting to move a lithium-ion battery.

“Experts say that the higher the battery charge, the greater the potential damage in the event of a fire,” a police spokesperson said in comments to Korea JoongAng Daily.

"Given testimony that the auxiliary power was not turned off, we will use forensic analysis to verify the sequence of events. Recovered batteries are undergoing disassembly testing, and identical models will be used in re-creation experiments to determine the cause of ignition.”

The fire destroyed 384 battery cells and multiple servers, and at one point, 709 government systems were offline as a result. As of Friday, 217 systems, 31 percent of the total, had been restored.

However, it is thought the government’s G-Drive, which contained 858TB of information, has been permanently destroyed in the fire, as no backup existed.