As they search for a motive, officers are hunting a white man seen running from a rented bedsit in south-east London moments after it exploded.

Laurent Bonomo and Gabriel Ferez, both 23, were found inside where they had been bound, gagged and stabbed to death.

A pathologist later documented 243 separate injuries to their badly-burned bodies. Mr Bonomo suffered 80 wounds after he had died.

One source said police officers were still unsure if one or more people were involved. But he added: "We expect to make an arrest sooner rather than later."

The detective responsible for the inquiry said the injuries were the worst he had ever seen and left everyone involved in the investigation "deeply shocked".

It is believed the two Frenchmen may have been tortured for several hours after being surprised by an intruder as they played a computer game.

Police said Mr Bonomo, of Velaux, and Mr Ferez, of Prouzel, were "entirely innocent", with no criminal background in France or the UK.

Detectives said a burglary at the flat six days earlier in which a laptop was stolen could be linked to the double murder.

Forensic experts have found no evidence of forced entry and one line of inquiry is that a stolen set of keys may have been used.