Badly burnt clothes were found at the scene of a rail cable theft, police have said.
About 20m (65ft) of cable was stolen in the Theydon Bois area of Essex at about 00:45 BST.
Officers believe the charred clothing - a blue coat and woollen hat - could belong to someone seriously burnt during the theft.
British Transport Police said officers feared someone had been "seriously injured".
Det Insp Andy Winters said: "After conducting an initial search at the scene, we recovered some badly charred clothing and are clearly very concerned that someone may have been seriously injured.
"In addition, we have the possibility, as a worst-case scenario, that somebody has been electrocuted and is currently unaccounted for.
"I would encourage anybody who has a friend or relative who may not yet have returned home, or who has unusual or unaccounted for burn injuries to contact us as soon as possible."