Winnenden, Germany - Experts on Tuesday continued to probe whether violent computer games influenced a teenage gunman who shot dead 15 people at his former school in the south-west of Germany last week.

A search of the youth's room turned up numerous computer games showing random killings and other violence, investigators said.

"Something was brewing there," said Heribert Rech, interior minister of the state of Baden-Wuerttemberg, where the March 11 shootings at the school in Winnenden took place.

Investigations also continued into whether the gunman, Tim Kretschmer, revealed his intentions hours before the massacre. Police initially said he did so on the internet, but later withdrew the claim after the entry on a website chat forum appeared to have been faked.

Experts were examining the computer of the boy's mother and were also questioning owners of internet cafes in the area to see if he had been there.

Kretschmer, 17, shot dead nine pupils and three teachers at the Albertville Secondary School with a Beretta pistol he stole from the bedroom of his father, a hobby marksman who owned 15 guns.

The teenager then shot dead three other people as he fled, before police stopped him in a town 40 kilometres away and he turned the gun on himself. An autopsy later confirmed he shot himself in the head.

Police have launched a criminal investigation into the father on suspicion of involuntary manslaughter.

In a statement issued through their lawyer, Kretschmer's family for the first time Tuesday expressed their condolences to the victims and their families.

"The appalling and incomprehensible action of our son and brother has robbed you of the most valuable and most important (possession), a dear person," said the statement from the parents and their teenage daughter.

"We never imagined Tim would do such a thing ... and saw him as a different person ... Our thoughts are also with those who suffered physical and psychological wounds," it added.

Hundreds of mourners attended the funerals Tuesday of two of the teachers, aged 25 and 26. One pupil was buried Monday and another on Saturday.

A memorial service for all those killed is due to be held in Winnenden on Saturday, with German Chancellor Angela Merkel and President Horst Koehler among the tens of thousands expected to attend.

Police said there had been 52 hoax threats against schools in Baden-Wuerttemberg since the shootings. Police were able to trace 39 of the culprits and issued arrest warrants for five of them, the state's interior ministry said.

The Winnenden shooting was Germany's worst school bloodbath since April 2002, when a 19-year-old high school student went on a rampage in Erfurt, killing 16 people before taking his own life.