It was shortly before 9:30 a.m., in the middle of the first break of the day when he strode into the schoolyard dressed in a long black coat and wearing a black gas mask.
"At first we all laughed at him, the way he was standing there. He looked totally ridiculous," said Dennis, a pupil in the seventh grade of the Geschwister Scholl secondary school in Emsdetten, a small town of 36,000 in northwestern Germany, near the city of Münster.
"Then he suddenly started firing and it scared us." Dennis ran to safety with his friend Jannick, 12. The bullets wounded a pregnant teacher, the janitor and seven pupils. Other people were injured through smoke inhalation from the smoke canisters he set off in his rampage around the school on Monday morning. None of the injuries are life-threatening.
Fellow pupils used to call Sebastian "Man in Black." "He was always totally dressed in black." He often hid his bright blue eyes behind sunglasses "even when the sun wasn't shining." "No one liked him," said Jannick. "But we didn't think he'd be capable of something like this. He somehow always seemed so shy, never laughed and always stood on his own in the schoolyard."
Sebastian killed himself at the end of his rampage and his body wasn't recovered until late on Monday night after experts had removed the explosive packs strapped to him. A total of 13 pipe bombs were found distributed around the school and in his car.
There's no school today and probably not tomorrow either but Dennis and Jannick aren't happy about it. "I'd rather never have a holiday again than go through something like this again."
Filled with hate
It's the same pattern as with the many other school shootings that have shocked communities around the world for years: Lonely teenagers obsessed with guns and violent computer games venting their frustration.
Sebastian was a good example. He was a gun freak who filmed himself posing in full combat gear wielding an array of weapons, setting off small bombs and carrying out a mock execution in the woods with a friend. He posted photos and video clips of himself in the Internet. He was due in court on Tuesday for illegal possession of a Walther P38 pistol.
Sebastian had planned his rampage long in advance, as a final showdown with all the people who he felt had humiliated him over the years. Guns blazing like in the computer video games he played, he wanted to eradicate himself and take his former fellow pupils and teachers with him.
"If you realize you'll never find happiness in your life and the reasons for this pile up day by day, the only option you have is to disappear from this life," he wrote in a farewell message he posted on the Internet. "The only thing I learned intensively at school was that I'm a loser," he wrote.
Peer pressure seems to have plagued him throughout his adolescence. He said he realized he lived in a "world in which money rules everything, even in school it was only about that. You had to have the latest cell phone, the latest clothes and the right 'friends.' If you didn't, you weren't worth any attention. I loathe these people, no, I loathe people," wrote Sebastian. "What's it all for? Why should I work? To break myself and retire at 65 and kick the bucket five years later?"
He entered his old school with two rifles with sawn-off barrels, a pistol, a gas-powered handgun, three bombs strapped to his body, a knife attached to his leg, 10 further homemade bombs and a petrol bomb in his backpack.
"It was clear he would flip out at some point, the way he used to talk," said a 17-year-old pupil who knew Sebastian. He was known as someone who spent all day at his computer playing games such as "Counterstrike", a game in which the player moves down corridors trying to shoot as many people as possible.
Obsessed with computer games
"He had a huge hatred of the world. Of his school, the teachers, everything," said the young man who declined to be named. "He was boiling with rage." He vented some of his aggression by listening to aggressive Death Metal music, and he dreamt of an army career. "He thought that was cool, going into battle in a uniform or combat gear. He liked to wear camouflage gear, no one objected to it."
In school breaks, Sebastian, generally known as Bastian, used to stand on his own, possibly because he was ashamed of having had to repeat two school years due to poor grades, and because he felt alienated from his new classmates.
In June 2004 he had posted a message in the Internet making his intentions clear. "This fear is slowly turning to rage. I am consuming all this rage and will let it all out at some point to take revenge on all the arseholes who wrecked my life! For those who haven't understood it exactly: Yes, this is about a shooting."
Yet he finished school with good grades in 2005 and even got an A in arithmetic. His rage remained, though. "Much of my revenge will be directed at the teachers, because they are people who intervened in my life against my will and helped put me where I am now: On the battlefield!" he wrote in his farewell note.
"I hate you and the way you are! You've all got to die! Since I was 6 you've all been taking the piss out of me! Now you're going to pay. Finally, I want to apologise for all this to the people who mean something to me or who were ever good to me! I'm gone "
Rudolf Egg, a criminal psychologist, described the letter as a "harrowing document" that marked a last appeal for help. It's unknown what relationship he had with his parents. He often went hunting with his father, a postman. Both collapsed after hearing the news and were treated in hospital for shock. His 16-year-old brother and 15-year-old sister are also receiving counselling.
In Berlin, the shooting provoked new calls for a ban on violent computer games, reigniting a debate that followed the Erfurt school massacre in April 2002 when former pupil Robert Steinhäuser shot dead 13 teachers, two pupils, a police officer and himself in Germany's worst shooting since World War II.
Gun controls were tightened in Germany after Erfurt with stricter checks on gun buyers. Pump-action shotguns with pistol grips were outlawed and the minimum age for gun purchasing was raised to 21 years from 18. Certain types of knives have also been banned.
Steinhäuser too was a games fanatic. Wolfgang Bosbach, deputy parliamentary group leader of Chancellor Angela Merkel's conservatives, said: "If it's true that the 18-year-old perpetrator intensively played so-called killer games, it's finally time for parliament to take action."
The interior minister of the eastern state of Brandenburg, Jörg Schönbohm, said: "Killer games make a fatal contribution to a growing tendency towards violence and they promote aggressive behavior. That's why strict action is needed against games that glorify violence."
The opposition Greens said the real problem had been the man's isolation rather than his obsession with computer games.
"We need to discuss this growing readiness to behave violently," said Jürgen Rüttgers, premier of the state of North Rhine-Westphalia where Emsdetten is located. "We can't allow our children to be confronted with violence in their daily lives."
"A lot of these students had a guardian angel yesterday and it will take them time to come to terms with what happened. One boy told me about how he slammed a classroom door shut and a bullet came flying though it."