Officers said they are "very concerned" for the well-being of someone who may be burned or suffering serious internal injuries, following an explosion in an electricity sub-station in Methil, Fife.
They believe the person could be seriously injured after trying to steal electrical cable at around 2am yesterday.
The thief tried to cut through a live mains cable which caused an explosion.
The power supply was cut and a number of houses and businesses also lost supply.
Inspector Donald Jenks, based at Levenmouth Police Station, said: "Investigations by ScottishPower have found that attempts had been made to cut through an 11,000 volt mains cable which subsequently shorted out causing an explosion and a failure of the supply.
"On examining the damage at the site of the incident, the Scottish Power engineer fully expected to find the person responsible to have been electrocuted.
"Police have conducted a thorough search in the area to ensure there is nobody in need of assistance.
"It is highly likely the person responsible for cutting this cable will have been badly burned or suffered an electrical shock; we would ask them to seek medical assistance as a matter of urgency.
"An electric shock of this magnitude could have caused significant internal injuries that might not be immediately apparent.
"Our initial inquiries are to ensure the safety of the person responsible, if you have any information that would help us identify this person please call us on 101."