Tim Kretschmer, 17, killed nine former schoolmates, eight of them girls, as well as three women teachers at the Albertville school in Winnenden, near Stuttgart. Seven other female pupils survived being shot.
Detectives were working on the theory that Kretschmer, described as a shy loner who never had a girlfriend, was motivated by a grudge against girls, possibly after being snubbed by classmates at the school which he left last year.
Heribert Rech, the interior minister of the Baden-Württemberg region, said: "It is striking that it was primarily girls who were shot. Eight of the dead students were girls, one was a boy. He wasn't shooting wildly or randomly when he entered the classrooms. He primarily shot to the head."
Kretschmer's only words during the two-minute shooting spree were heard when he returned to the classroom where he had first opened fire: "Aren't you all dead yet?"
Police said there would have been more deaths had it not been for the extraordinary sacrifice of three teachers, who stood in Kretschmer's way to protect their students.
Witnesses described how one of the women shielded a pupil while she looked defiantly into the killer's eyes.
"He gunned her down with ice-cold efficiency," a police spokesman said.
The killer managed to evade capture for three hours, hijacking a car and killing three more people before turning the gun on himself when he was finally cornered by police.
Angela Merkel, the German Chancellor, described the murders as "incomprehensible", adding that "it is a day of mourning for the whole of Germany".
The massacre, the second-worst at a German school, reopened the debate on the country's liberal gun laws: Kretschmer used a high-powered automatic pistol which was legally owned by his father, a prominent local businessman and gun club member. It also left parents struggling to comprehend why an increasing number of young boys have become mass murderers, following a string of high school killings across Europe and America in recent years.
Former classmates described the killer as a "gun freak" who spent his spare time shooting air weapons in a forest behind his home.
One female pupil said: "He was always a weirdo, obsessed with 'death metal' music and guns. He never had a girlfriend. There was some trouble at the school one time over his bullying. He was an angry guy, a loner. He hated the world. No one was sorry to see the back of him when he left."
Mr Rech said the gunman, a trainee salesman, was "shy, reserved, sporting and keen on weight training. Teachers described him as not particularly academically successful".
His rampage began at 9.30am when Kretschmer, dressed in black combat fatigues and wearing a gas mask, went into a chemistry class and opened fire.
Around 1,000 police officers were deployed in the subsequent manhunt, which left the town of 27,000 people in the grip of terror for three hours.
Police said that Kretschmer was eventually shot in the leg during an exchange of fire with plain-clothed officers, and appeared to have then shot himself.
Detectives were questioning his parents and examining his computer for any clues to his motive or any form of suicide note.
In several recent high school shootings, the killers have recorded films of themselves which have either been posted on the internet or sent to newspapers.
