Black smoke billows from an underground tunnel in Niiza, Saitama Prefecture, where power transmission cables caught fire on Oct. 12, causing a blackout that affected more than half a million households in Tokyo. (Video footage by Kenji Notsu)

The fire that caused a blackout affecting more than half a million households in Tokyo may have been sparked by deteriorating underground cables that had not been replaced in the 35 years since they were laid.

Tokyo Electric Power Co officials believe the fire that broke out at about 2:55 p.m. on Oct. 12 in Niiza, Saitama Prefecture, just north of Tokyo, could have been ignited by a spark from the worn cables.

Police and fire department officials began an inspection of the site on Oct. 13, and TEPCO is also investigating the cables.

The power blackout temporarily affected about 586,000 households in a wide area of the Japanese capital. Power was restored to all the households by around 4:25 p.m on Oct. 12.

However, the fire at Niiza was not completely extinguished until after midnight.

A visual inspection of the power transmission cables is conducted twice a year, but TEPCO officials said there were no strict rules about their operating life.

Cables are repaired if irregularities are found, which had not happened in this case.

Saitama prefectural police said the fire had been extinguished at 12:21 a.m. on Oct. 13. Police officers and firefighters began checking the safety of the site later in the morning.