A series of blunders left a violent criminal free to torture and murder two French research students at a flat in south-east London.
Dano Sonnex and accomplice Nigel Farmer were jailed for life for murdering Gabriel Ferez, 23, and Laurent Bonomo, 23, in New Cross in June 2008.
The students suffered hundreds of stab wounds and their flat was set on fire.
Justice Secretary Jack Straw apologised for errors, but the victims' families said they would sue UK authorities.
Sonnex, 23, must serve a minimum of 40 years. Farmer, 34, was told he must stay in prison for a minimum of 35 years.
The students' parents said they were going to sue British authorities for a "failure" to prevent Sonnex carrying out his brutal attacks.
Olivier Ferez, Gabriel's father, said an apology from Mr Straw "will not suffice" and his lawyers were now involved.
Guy Bonomo, Laurent's father, said the two students "would be alive today if the British justice system had not failed us".
It emerged after the trial that a catalogue of failures had led to Sonnex being on the loose.
Father: Killers 'were animals'
Sonnex was bailed for handling stolen goods when he should have been remanded in custody by magistrates.
Further blunders delayed his recall to prison, and it took 33 days for the administrative process to be completed. It was a further 16 days before Metropolitan Police officers went to Sonnex's house to arrest him, by which time the two young French students were already dead.
Earlier, Sonnex had tied up and threatened a couple, and could have been returned to jail, but he was given a verbal warning by his probation officer rather than being the subject of a review.
Mr Straw apologised for probation service errors which allowed Sonnex to kill, and London's chief probation officer David Scott has resigned.
Mr Straw said: "The direct responsibility for these killings must lie - as the jury found - with the criminals Sonnex and Farmer.
"But it is also the case that Sonnex could and should have been in custody at the time he committed these murders.
"It was the consequence of very serious failures across the criminal justice system that he had not been arrested and incarcerated some weeks before."
What the firemen found was a scene of almost unimaginable horror
Mr Straw added that a number of reviews had since taken place, the result of which was that 100 new probation officers would be recruited in London over the next two years.
In sentencing, trial judge Mr Justice Saunders said: "I am satisfied that the only possible reason for the number of stab wounds is that the killings were sadistic.
"The killers got pleasure from what they were doing."
After sentencing, Guy Bonomo, Laurent's father said: "We have not seen the trial of two human beings - they are animals.
"We were hoping for a more severe sentence - they should never be let out."
Francoise Villemont, the mother of Gabriel Ferez, said: "Nothing will remove the suffering and I can never accept the torture that was inflicted on Gabriel gratuitously."
Farmer was also found guilty of burglary, a charge Sonnex had already admitted. Both men were also convicted of false imprisonment and arson.
During the trial, prosecutor Crispin Aylett QC said Mr Ferez and Mr Bonomo had been tortured then murdered because Sonnex and Farmer were unable to get cash from a machine using one of the students' bank cards.
He said the students had been stabbed 244 times by Sonnex and drug addict Nigel Farmer, 34, during a three-hour torture ordeal.
Sonnex and Farmer had blamed each other, but Mr Aylett described the crime as a "joint enterprise of unmitigated evil".
Mr Bonomo and Mr Ferez were biochemistry students at the Polytech Clermont-Ferrand.
They were weeks away from finishing a research project at Imperial College when they were killed.
Farmer and Sonnex were caught by CCTV several times on the day of the murders
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