He admitted to stealing 239 artworks and other exhibits from 172 museums while travelling around Europe and working as a waiter, an average of one theft every 15 days.

He was a self-described art connoisseur who stole in order to build a personal collection of stolen works, particularly of 16th and 17th century masters.

[7] According to journalist Michael Finkel's 2023 book The Art Thief, Breitwieser's first theft was in early 1994 in Thann, a medieval town in northeastern France.

With his girlfriend keeping watch, Breitwieser worked out the nails holding the painting in its frame and slipped it under his jacket.

[9] He would typically visit small collections and regional museums, where security was lax, and Kleinklaus would serve as his lookout as he cut the paintings from their frames.

[1][2] The single most valuable work of art he stole was Sybille, Princess of Cleves by Lucas Cranach the Elder from a castle in Baden-Baden in 1995.

[10] Breitwieser did not attempt to sell any of his large collection of art for profit at first; instead he enjoyed thinking about how he was "the wealthiest man in Europe."

[2] His mother, Mireille Breitwieser (née Stengel), thought the works had been bought at auction and only later suspected that he had not acquired them legitimately.

[10] In November 2001, he was caught after stealing a bugle dating from 1584, one of only three like it in the world and with an estimated value of £45,000, from the Richard Wagner Museum in Lucerne, Switzerland.

[10] When Mireille Breitwieser heard of her son's arrest from Kleinklaus, who had been able to evade Swiss authorities, she proceeded to destroy many of the works Breitwieser stored at her house in Mulhouse: contemporary reports suggested she cut or carved them up, leaving the remains of the frames in the trash over several weeks and forcing the shredded paintings down her garbage disposal unit,[1] but, as most of the paintings were on wooden panels, it seems more likely that they were, as she confessed, incinerated in a pyre in nearby woodland.

[11] She threw other stolen artifacts, such as vases, jewelry, pottery, and statuettes, into the nearby Rhône–Rhine Canal, where a few later washed up on the shore; most of the 107 pieces were recovered through dredging and diving work.

[citation needed] His mother admitted to destroying the artwork some seven months later, after some pieces had washed up on the banks of the Rhine.

[9] In 2005, editor of the cultural section of the French newspaper Libération Vincent Noce published a book about Breitwieser and the investigation into his thefts, titled La collection egoiste (The Selfish Collector).