Two French research students found stabbed to death following a flat fire had been tied up and suffered horrific, excessive injuries, police have said.
The bodies of Laurent Bonomo and Gabriel Ferez, both 23, were found in a ground-floor flat in New Cross, south-east London, on Sunday night.
They had suffered a total of 243 stab wounds
to the head, neck and chest before being set alight.
The bio-chemistry students had been at Imperial College for two months.
Det Ch Insp Mick Duthie said he had no idea why the students were killed.
Speaking at a news conference at Scotland Yard, he said the flat had been broken into six days before and a laptop had been stolen.
Police said they were shocked by the crime
He also said a white male was seen running from the scene of the fire, but police have not ruled out the possibility others might have been involved in the murders.
The students' neighbour told the BBC: "A fire was shocking enough, but when we came back the next day and were told that it was a murder scene, well no-one welcomes that.
"My wife was very upset, it's very shocking."
Mr Duthie said one line of inquiry was that the attack was a case of mistaken identity.
"We cannot find anything in these two young men's background to suggest they were involved in any criminality, that they had done anything wrong," he said.
A London Fire Brigade cordon outside the flat where French students Laurent Bonomo and Gabriel Ferez were found dead
The two bodies were found in the ground-floor flat
The BBC's crime correspondent Ben Ando said Mr Bonomo had 196 stab wounds, including 100 to his back, and Mr Ferez had been stabbed 47 times.
He added a chemical was used as an accelerant in the fire.
Mr Duthie said: "The extent of the injuries are horrific.
"Everyone working on this case, including myself, has been deeply shocked by what we have seen.
"I have never seen injuries like this throughout my career.
"I would not say this was a professional attack. I would say it was a frenzied, horrible, horrific attack.
"I imagine it would take some considerable amount of time to inflict the nature of the injuries."
The detective said no weapon had been recovered during extensive searches in the area and police were studying some "limited" CCTV footage.
Mr Duthie said the students had been in London since the beginning of May and were due to return to France at the end of July.
Imperial College's Rector, Sir Roy Anderson, said: "Laurent and Gabriel had bright futures ahead of them and it is dreadful that their lives should end so soon."
The emergency services were called after residents reported an explosion in Sterling Gardens on Sunday night.
Mr Ferez, who lived at another south London address, was visiting his close friend Mr Bonomo when the attack took place.
Mr Duthie said Mr Bonomo was known to be alive in the early hours of Sunday morning because he spoke to his girlfriend in France.
The parents of the two men are in the UK and have had to identify their sons, police said.
Police said Mr Bonomo, of Velaux, and Mr Ferez, of Prouzel, studied in Clermont-Ferrand, central France, and had been chosen to take part in a three-month research project into chains of protein within DNA at Imperial College.