The scandal, which has centred on an as yet unnamed 20-year-old cavalier with the German national team, erupted days before the world equestrian championships in Tryon in the United States.
He is suspected of “sexually abusing, alone or with accomplices, young women during tournaments” in Germany and abroad, said a statement from prosecutors.
German daily Der Spiegel claimed a group of male riders got their victims drunk at tournaments in a bid to facilitate the sexual abuse.
The report said prosecutors are probing claims one victim was abused at the European championships in Samorin, Slovakia.
'Hardly a surprise'
Sönke Lauterbach, the chief director of the German Equestrian Federation (FN), confirmed one 18-year-old rider had already been handed an 18-month ban for his role in the abuse.
“There's a group (or riders) whose behaviour, especially with alcohol, is extremely bad,” said Lauterbach.
“We all know about it, but I believe we're talking about an isolated group.”
For German equestrian legend Ludger Beerbaum, the claims were hardly a surprise.
“We've closed our eyes for too long,” the four-time Olympic champion said in comments to SID news agency, a subsidiary of AFP.
“It starts with the parents, then the coaches and then those who are in charge of the federation. All of them have to accept responsibility now.”
He added: “Those found using alcohol should be thrown out of tournaments. And if there is repeat behaviour, they should be handed a lengthy ban.”