Eislingen, Germany - A 19-year-old youth has admitted to taking part in the killings of four members of a family in south-west Germany, prosecutors said on Friday.

The teen told police that he and his friend, 18, gunned down the friend's parents and two sisters at the family's apartment in Eislingen a week ago, a spokesman for the prosecutor's office in nearby Ulm said.

The youth did not give a motive for the killings, which were carried out with two pistols stolen last year from a local marksmen's club frequented by the 18-year-old.

The guns and 31 rounds of used ammunition were found in a woods wrapped in a plastic rubbish bag. The 19-year-old said his friend had hidden them there, according to police.

Police said the duo behaved like professional killers. They first shot dead the two sisters, aged 24 and 22, while their parents were out for the evening with friends in a local restaurant.

They then went to the restaurant themselves and joined the parents, returning to the apartment an hour later to wait for the couple, aged 57 and 55, to come back.

An hour later, in the early hours of April 10, they shot dead the two adults in the hallway of the flat, police said.

The 18-year-old has remained silent about what happened since he and his friend were detained for questioning a day after the killings.

Investigations showed traces of powder burns on the hands of both youths, indicating they had recently fired guns.

The killings were reported to police by 18-year-old, who said he found the bodies of his family when he returned home after spending the night with his friend.

Police immediately suspected a family member or someone with close links to the family because there were no traces of a break-in and the flat was not ransacked.

The father practiced alternative medicine in the basement of the apartment building, which was owned by the family. The mother was a teacher and her daughters were studying to be teachers.

The killing occured three days after a man opened fire in a court in Bavaria, killing his sister-in-law and injuring two other people before turning his gun on himself.

That shooting happened less than a month after a teenager went on the rampage in and around his former school in Winnenden, south-west Germany, killing 15 people. (dpa)