LONDON —  The parents of a French student savagely stabbed to death in London called Sunday for his killer or killers to turn themselves in, and urged the British people to help police solve the crime.

Police released without charge the only person so far arrested in connection with the June 29 deaths of Gabriel Ferez and Laurent Bonomo.

The men, both 23-year-old graduate students, were bound and stabbed to death at Bonomo's rented apartment. Bonomo had been stabbed nearly 200 times, Ferez nearly 50 times. Their bodies were then set on fire.

Ferez's parents, Francoise and Olivier Ferez, said their son's attackers would not be able to hide forever, and called on them to "recognize this terrible mistake you made."

"Rest assured that we will not leave you in peace," the couple said in a statement.

The gruesome killings have spurred soul-searching in a city already alarmed by a rising number of knife attacks. Eighteen teenagers have been violently killed in London this year, compared to 27 in all of 2007. Most were stabbed, and police in the city say the fight against knife crime has overtaken terrorism as their top priority.

Police arrested a 21-year-old man Saturday over the killings, but freed him the next day and said no further action would be taken against him.

On Sunday, police released a computer-generated image of a man several witnesses saw running from the apartment building the night of the attack. He was described as slim, white, between 30 and 40 years old and wearing jeans, a dark top, white running shoes and a baseball cap.

Ferez's parents urged witnesses to come forward.

"Tell the police anything you know; do not be afraid," they said. "Do not feel useless or like an 'informer.'

"Someone, somewhere must know something that will help to solve this terrible crime.

"We ask our friends, the British people: please help us," they added.

Both the French victims were promising bioengineering students taking part in a three-month DNA research project at Imperial College in London, one of Britain's top universities.

Police have said the murders may be linked to the theft of two PlayStation game consoles, which they believe were taken from the apartment the night of the killings. A laptop computer also was stolen from the apartment six days before the attack.