A radiation alarm went off at Tokyo Electric Power company’s (TEPCO) Fukushima power plant even before the March 11 earthquake and tsunami, which served as a contrary to earlier theories the reactors were damaged by the natural catastrophes.

A supervising post on the border of the plant went off minutes before the station was hit by the tsunami that damaged the backup power that kept reactor cooling systems running. The monitor was programmed to go off during high radiation levels, the company said.

TEPCO’s head of nuclear facility management Teruaki Kobayashi said investigations are still in progress if the monitoring facility was working properly and there is a possibility radiation leaks happened before the tsunami arrived.

The company said it didn’t have the precise radiation reading which triggered the monitor’s sensor.

Officials at TEPCO earlier said the Fukushima facility withstood the massive earthquake but was damaged by the tsunami that followed which caused the worst nuclear catastrophe since Chernobyl in 1986. The early radiation alarm, caused by the earthquake, may prove to be vital as other reactors in Japan had safety upgrades which are only focused on threats from tsunamis.