In the early 1980s, he formed a close partnership with John Mieremet, initially as part of a gang called De Denkers.
Klepper joined the Amsterdam chapter of the Hells Angels and rose up to the rank of "prospect" (the second level in an outlaw biker club).
[1] The Canadian journalists Julian Sher and William Marsden described Klepper as "an infamous and vicious Dutch bank robber, arms dealer and drug trafficker who had made more than enough enemies in his time, but nobody had ever dared to touch him because he had the backing of the Hells Angels".
[1] Klepper's bodyguard set off in a wild chase through the streets of Amsterdam and though a marketplace, exchanging shots with the assassin.
[1] The president of the Amsterdam chapter of the Hells Angles, Willem van Boxtel, gave Klepper the posthumous promotion to a "full patch" member.