The 44-year-old - who won the Tour back in 1997 - allegedly had a dispute with a female sex worker at a hotel. and the incident turned physical.
"Jan Ullrich was arrested today after a dispute at a hotel in Frankfurt", a police spokesperson explained.
Police and local prosecutors later said in a statement that they were investigating a case of attempted manslaughter and grievous bodily harm but that the suspect, who they did not name, had been released.
But he then got into a quarrel with the "escort lady" and "physically attacked and injured her", police said. "Mr. Ullrich is still in custody".
After spending the night with this woman, the only German victor of the Tour de France hit her and strangled her, authorities said.
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The woman was still being questioned by police and the investigation was ongoing.
Earlier this week, the Ullrich opened up to German newspaper Bild about his separation from his wife and children.
"I have done and taken things as a result that I very much regret".
Ullrich, the only German to win the Tour, was dogged by doping allegations throughout his career and admitted to the German magazine Focus in 2013 that he received blood-doping treatment from the Spanish doctor Eufemiano Fuentes, adding: "Almost everyone took performance-enhancing substances then".
Ullrich served a two-year ban for doping in 2012, five years after he retired in 2007.