The exact cause of the huge blaze at a Grantham substation last week, could take weeks to be identified.
Lincolnshire Fire and Rescue said today that it is currently being treated as an unknown electrical fault within the substation.
Western Power Distribution have also been investigating at the scene of the fire, on Spring Gardens, and added that it could take weeks before the exact cause is known.
The substation exploded into flames on Tuesday, cutting electricity to just under 13,500 homes and businesses, and sending black smoke hundreds of feet into the sky.
Five crews from Lincolnshire Fire and Rescue were called to the substation, which unexpectedly exploded, and risked spreading to a second substation and electricity pylon. They received further support from fire teams based at RAF Cranwell and RAF Waddington.
The blaze caused widespread disruption, including to the East Coast Mainline, while surrounding roads were also closed by Lincolnshire Police.