LONDON, July 6, 2008 (AFP) - The parents of one of two French students savagely killed in London vowed to pursue their attacker Sunday, as police stepped up their search one week on.

'Rest assured that we will not leave you in peace,' Gabriel Ferez's parents Francoise and Olivier said in a message to the killer or killers, who bound the two students and repeatedly stabbed them to death.

A week on from the murders of 23-year-olds Ferez and Laurent Bonomo in south-east London, police have yet to announce a motive for the crime, the time of death or whether the attack was the work of one culprit.

Officers stopped pedestrians and drivers Sunday night in the cul-de-sac in New Cross where the bodies of the brilliant academics were found in a burnt-out flat a week ago.

Bonomo is thought to have been stabbed nearly 200 times while Ferez suffered around 50 wounds in a prolonged ordeal. Their bodies were found bound up.

Speaking at the scene, Detective Chief Inspector Mick Duthie, who is leading the investigation, told reporters that officers are still not sure exactly when the friends -- both biochemists studying in London on an exchange -- died.

'Laurent spoke to his fiancee around 1am (0000 GMT) on the Sunday morning,' he said.

'After that, no one heard from Laurent or Gabriel or saw them. It is important to stress that the attack could have taken place at any time during Sunday.

'We believe that anyone involved in this scene would have been bloodstained when they left the area.'

The bodies were found late last Sunday night after emergency services were called to a fire and explosion.

Earlier, police released 'with no further action' a 21-year-old man arrested in the street early Saturday.

They issued an e-fit of a man they said was seen running away from the scene described him as 'white, between 30 and 40 years old, of slight or slim build and wearing a light-coloured baseball cap and a dark top, blue jeans and white trainers'.

Duthie appealed for anyone who knew the man in the e-fit or who saw anything unusual near the scene that night to come forward.

Earlier Sunday, Ferez's parents told the killer in a statement: 'You will not be able to live in hiding forever. You may be scared and feel like a coward, but you must recognise this terrible mistake you made.

'Rest assured that we will not leave you in peace.'

One theory is that the deaths were linked to a burglary at the flat, rented by Bonomo, on June 23 in which a laptop was taken.

Police believe two Sony PSP games consoles have since gone too and the victims' bank cards are missing. They are in touch with French banks to see if the cards had been used since the attack.

Duthie said officers were looking at forensic material from the first burglary and appealed to whoever was behind that to come forward.

He said that there was no evidence that keys to the flat had been taken in the burglary and said police had not yet found the pair's mobile phones.

The students had been due to return to their university in Clermont-Ferrand, central France, later this month.

Their deaths coincided with growing concern about the extent of knife and gang culture in London, which newly elected Conservative mayor Boris Johnson and Metropolitan Police chief Ian Blair have vowed to tackle.

A total of 18 teenagers alone have been knifed to death so far this year.