The parents of brutally murdered 16-year-old au pair, Sophie Trezzini, expressed relief on Thursday over a sentence of mandatory life imprisonment handed the killer by Aargau’s supreme court.

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The appeal ruling toughened a conditional life term issued by a lower court in February against the 29-year-old Aargau man convicted of killing the woman from the canton of Fribourg at his apartment in March 2009.

The prosecution appealed the lower court’s sentence, which could have allowed for parole after a certain period.

“We have achieved something for society,” Trezzini’s mother said to reporters in an emotional declaration after leaving the courthouse.

However, a lawyer for the killer said he would appeal the sentence to the federal supreme court.

With its lurid details, the sensational case has attracted nationwide media attention.

It dates back to March 4th 2009 when the convicted man, an unemployed cook identified as Daniel H., met the au pair — who was working for a family in Schwyz — at the Zurich train station.

He lured the girl to his apartment in Rieden bei Baden (Aargau) on the pretext of a photo shoot to model jewellery, promising her 500 francs, according to earlier court testimony.

The prosecution said Daniel H. bashed her in the skull with a barbell and slit her throat with a knife.

There was conflicting evidence over whether he had sex with the girl, although it was found that the killer interfered with the dead body.

He surrendered to police several days later.

The killer had a previous record for attempted murder dating from 2004.

At issue in the case was whether the man, who expressed regret for the murder and said he acted under the influence of drink and drugs, was capable of rehabilitation through therapy.

The president of the three-judge court declared in a statement that the killer was “not currently treatable.”

This view was based on the evidence provided by two psychiatrists.

In announcing an appeal, a defence lawyer for Daniel H. indicated to the SDA news agency that the country’s top court could clarify what was meant by being “permanently” beyond the help of therapy.

Life imprisonment has been enshrined in Switzerland’s criminal code since an initiative was approved by Swiss voters in 2004 aimed at sentences for violent criminals and sex offenders considered untreatable.

A “simple” life sentence is indefinite, allowing for parole after 10 or 15 years, with its continued necessity subject to regular review.