The bodies of Laurent Bonomo and Gabriel Ferez, both 23, were found in a house in southeast London after neighbours heard an explosion and saw a man running from the scene.
The Metropolitan Police said the biochemistry students working on a DNA project were covered with stab wounds all over their bodies.
The friends were dead before the fire started at Bonomo's rented flat in New Cross late on Sunday.
"The extent of their injuries was horrific," said Detective Chief Inspector Mick Duthie. "Everyone working on this case has been deeply shocked by what they've seen. I've never seen injuries like this throughout my career.
"The level of violence used on these two victims was excessive, it's horrendous".
Officers have no idea why they were attacked and appealed for anyone who knew them to come forward. They had no criminal links or contact with the police in France or England, Duthie added.
"They are of good character. We cannot understand why these two young men were targeted," Duthie said.
Police are looking at the possibility that they were the victims of mistaken identity. Someone had broken into the flat six days before the fire and stolen a laptop computer.
The pair were on a three-month research project at Imperial College London, one of the world's leading science universities.
They had arrived in London in May to study DNA at Imperial and were due to return to France later this month. Media reports said they had come from Clermont-Ferrand, central France.
Their families travelled from France to identify their bodies on Thursday.
No weapon has been found. Police are searching the area for clues and have seized security camera footage. The London Fire Brigade has launched an investigation into the blaze.
