Firefighters found the teenager's body 10 days ago in a burning, empty flat under renovation. The 45-year-old suspect strangled her with a cable, then started the fire to cover his tracks, prosecutors said.
The attacker spent 18 years in prison for nine rapes, three attempted rapes and a sexual assault. He was released in 2016.
Rachida Dati, justice minister from 2007 to 2009 under president Nicolas Sarkozy, told Europe 1 radio: “These kinds of criminals must not be left to go free.”
Dati put into place a preventive detention system in 2008, according to which an individual remains detained – even after they have finished serving their prison sentence – if they are considered dangerous.
“The Hollande government completely weakened (…) this measure and the current government has not put it back into place,” she said, referring to the 2012-2017 Socialist administration of president Francois Hollande who succeeded Sarkozy.
“The justice system is corrupted by the far left and its culture of excuses,” deputy-head of the far-right National Rally party, Jordan Bardella, told LCI television on Monday.
“When you represent a threat for French society, as was the case, you should not be allowed to leave prison,” Bardella said.
Critics say the system poses legal and fundamental rights problems as it amounts to imprisoning people for crimes that have not been committed.
No recent element indicated that the suspect might re-offend, Nantes deputy prosecutor Yvon Ollivier told a press conference Saturday.
After leaving prison, the suspect moved, got a job and found a partner. He saw a psychotherapist once every two months, Ollivier said.
“When there is such a tragic event, it is a failure for everyone,” he said.
The suspect was arrested on Saturday and currently being held in detention.
An investigation by France's General Inspection of Justice, which monitors the work of the judiciary, is underway to determine whether the individual had been monitored properly since prison.