Eckehard Weiss, who coached Tim Kretschmer at table-tennis, told the BBC that he had been a relatively normal boy.
Investigators are now trying to find out why Kretschmer confessed being "fed up with his horrid life" and what triggered the shootings.
Officials said Kretschmer had graduated from Albertville school with average grades in 2008 and had been studying sales at a vocational school since.
Hansjorg Kretschmer is the ambassador for the European Commission to Turkey.
"For fun, because it's fun, " Kretschmer replied, speaking very loudly.
There has been speculation that Kretschmer - described by some of his former classmates as "a loner" who never had a girlfriend - could have held a grudge against his school.
Ambassador HANSJORG KRETSCHMER (European Commission): We should not forget that what Turkey is trying to achieve in these years is rather a revolutionary change of attitudes which have been ingrained in Turkey for many decades.
He said Kretschmer had attended five sessions between April and September last year at a clinic in the town of Weissenhoff, and was expected to continue outpatient therapy in Winnenden, but broke off the treatment.
This large and competent study, by professors Nick Bloom, Toby Kretschmer and John van Reenen, was actually read by some, be it clearly without a proper understanding of its methodology (which, indeed, it being an academic paper, is hard to fully appreciate without proper research methodology training).
FORBES: 'Can't Believe It' (We Deny Research Findings That Defy Our Beliefs)
Fortunately, recently, professors Nick Bloom, Toby Kretschmer, and John van Reenen (from Stanford, the University of Munich, and the London School of Economics) conducted an extensive study examining the effect of family-friendly practices on hard variables such as firm sales per employee and return on capital employed, using a large database of firms from the US, the UK, Germany, and France.
FORBES: Are Family-Friendly Workplace Practices Worth Their Money? New Evidence
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