FREUDENSTADT (Germany )— In July 2015, star students at the Heinrich Schickhardt vocational college in Freudenstadt were treated to a little homespun wisdom from the deputy head. Everyone, said Ms Ursula Wolf, must find their own way in life: “It does not matter if you take a straight road to your goal, or a winding path — as long as you move in the right direction”.

If German prosecutors are to be believed, one of her students took a path so winding, he ended up in prison.

Sergej W, who was awarded a prize that day for one of the best exam results of the year, was arrested last month on suspicion of bombing the Borussia Dortmund football team just minutes before a crucial European fixture.

It is a case without precedent in Germany, and one that has shocked the country to its core. Prosecutors claim the attack was motivated by greed. They say that on the day of the explosion, Sergej W — German authorities do not reveal a suspect’s full name — bought thousands of put-options on Borussia Dortmund shares, betting they would slump.

They did not, and a few days later, as he arrived at work in the southern city of Tubingen, the 28-year-old Russian-German was apprehended by the police.

Held in a high-security prison in Stuttgart, he is being investigated for attempted murder, and causing an explosion and grievous bodily harm. Friends and acquaintances are horrified. But they also recall conversations with him that now are tinged with a new meaning.

“He would say to me that he did not just want to work his whole life, that he wanted something more,” says Mr Andreas, who knew him when they were both doing an apprenticeship in Freudenstadt, the suspect’s hometown in the southern state of Baden-Wurttemberg.

Described in the German press as the “Black Forest Phantom”, Sergej W comes across as a shadowy figure, an enigmatic loner who sought companionship in a local Christian group but, according to prosecutors, eventually succumbed to the promise of easy money — even at the price of a spectacular bomb attack that could have killed several of Germany’s best-paid football players.

Sergej W himself has given no clues. For days, he refused to say anything to his interrogators. Then he finally spoke. “He told them he did not do it,” says Mr Reinhard Treimer, his court-appointed lawyer.

Prosecutors portray him as a cold-blooded schemer who combined his skill in electronics with the financial literacy of a retail investor — to devastating effect. He was identified by pure chance.

A vigilant bank employee noticed strange movements in derivatives of Dortmund shares on April 11 — the same day three bombs went off next to the team’s bus as it ferried players to a Champions League home match against French side AS Monaco.

Sergej W was born in April 1989 in Chelyabinsk, a drab industrial city in the Russian Urals. It was a region that had become home to thousands of ethnic Germans deported eastward in 1941 by Joseph Stalin, who feared they would collaborate with the invading Nazis.

After the iron curtain fell, a wave of these “Russian-Germans” began returning to their ethnic homeland. Sergej W’s family joined the exodus, moving to Germany in 2003, and eventually settling in a modern block of flats near the train station in Freudenstadt, a pretty medieval town in the Black Forest. “They are an ordinary family, good and decent,” says Mr Ahmet Teker, the building’s janitor.

The suspect, he said, “did not stand out in any way”.

He learnt German and went to secondary school, later doing an apprenticeship as an electronics engineer, combined with study at a local vocational college, the Heinrich Schickhardt school.

His training was at Schmid Group, a company based in Freudenstadt’s small industrial zone that makes circuit boards, photovoltaic panels and automation systems. “He completed his course in three years rather than the usual three-and-a-half,” says a senior college official. “He was a top-performing student.” But he was also different from his classmates. “He was a lone wolf,” says the official. “He could work very well on his own, but he was not a team player.”

Mr Harald Klager, who ran a cafe in the school, says he did not seem to have any friends. “He was calm, but there was a certain reserve about him,” he says.

Mr Andreas, a fellow apprentice who asked that his real name not be used, describes Sergej W, who retained a strong Russian accent even after 12 years in Germany, as a creature of habit, who “every lunch break would sit there eating his sandwich, apple and croissant”.

Their lunchtime conversations ran the usual gamut of young, male preoccupations: Sports betting, company gossip, gyms, films and TV series, “sort of the meaning of life”, Mr Andreas says.

Like many of his generation, the suspect was keen on computer games such as Outlast 2 and Grand Theft Auto V, which let players go on a violent crime spree. But he was not very sociable. “He was unassuming and often alone,” says Mr Andreas.

Yet his Facebook page has an air of escapism. A profile picture shows him against a backdrop of classical ruins, sun-tanned, blond hair parted, wearing sunglasses and a Hawaiian shirt. It is an image that is hard to reconcile with the humdrum job he took after his studies — as an electrician at a power plant in the quiet university town of Tubingen.

Sergej W has only three likes on Facebook: The African pop group Terdamite, Freudenstadt Volksmission — a Pentecostalist church — and LifeGuard, its youth affiliate.

Mr Christoph Fischer, pastor at the Volksmission, describes him as a “sporadic visitor” to services and youth events. A photo from 2014 shows him with a group of other young Christians baking Christmas cakes.

Prosecutors claim the young Russian-German set his plan in motion on April 9, when he checked in at the L’Arrivee hotel in Dortmund, the same one used by the eponymous Bundesliga team.

Using the hotel’s Wi-Fi, he bought thousands of put options in Borussia Dortmund shares from online bank Comdirect. These would have yielded a profit of several hundred thousand euros if the shares had crashed — which they might have done if several players had been killed.

Prosecutors believe he installed three bombs stuffed with metal pins in a hedge near the hotel and detonated them, possibly using a mobile phone, as the Dortmund team bus drove past. The blast was so powerful that one metal shard was found 250m away from the bus. A player, Marc Bartra, as well as a policeman were hurt.

Hotel staff noted that while other guests in the L’Arrivee’s restaurant ran in panic, Sergej W calmly ordered a steak. But his bet failed: The Borussia share price did not fall. It even ended up on the day after the attack, closing 1.8 per cent higher at €5.71 (S$8.70) after a brief dip.

Meanwhile, Comdirect had noticed his unusual trades and tipped off the Financial Intelligence Unit of the Bundeskriminalamt, the federal police agency. A Comdirect spokeswoman declined to comment, but said that “like any bank, we are required by Germany’s laws to report any transaction arousing a suspicion of money-laundering or terrorist finance to the relevant authorities”.

According to a reconstruction of his trades by Die Zeit, Sergej W had acquired a total of 116,000 put-options and other instruments over a three-week period, at a cost of €10,208. When he saw Dortmund’s stock had not swooned as expected, he sold most of the derivatives, incurring a loss of €2,040.

Although the attack was well-planned, Sergej W had, according to investigators, made a number of key mistakes. The second of the three bombs was placed too high off the ground to have much of an effect. The three letters he left at the scene claiming responsibility, each containing the words “In the name of Allah”, were amateurish and unconvincing.

And he used the IP address of the L’Arrivee to do his deal, leading police straight to him. Yet prosecutors still have much work to do to prove his guilt.

Investigators believe he had his own amateur bomb-making laboratory — but have yet to find it. And there appears to be no hard evidence connecting him to the explosives used in the attack. Despite that, the authorities felt they had enough proof to detain him, which they did on April 21.

Fearing he might have set up booby traps, they did not take him at his home, in the small town of Rottenburg am Neckar, but as he arrived for work in Tubingen. Mr Andreas, his fellow apprentice, is still struggling to make sense of the story. “I still cannot understand why he would have such delusions of grandeur — and be so greedy,” he says. FINANCIAL TIMES