A security firm says it detected movement at Sungnyemun just before Sunday night's devastating fire.

KT Telecop, the security company entrusted with the Joseon Dynasty wall gate, reported that its on-site infrared sensors picked up movements at 8:47 p.m. Sunday. After they picked up movements three minutes later that triggered an alarm, the company sent security guards to the site.

KT Telecop added it had set up six infrared sensors around the number one national treasure.

The fire erupted at 8:48 p.m., right after the first alarm was set off. However, since the firm had no closed-circuit surveillance camera near the sensors, it did not have images of any arson suspects.