Spanish police on Monday arrested Somali track coach Jama Aden, whose Ethiopian protege Genzebe Dibaba is the current world 1500m champion, as part of an investigation into drug trafficking.
The dawn arrest came at a hotel in the Catalonian town of Sabadell occupied by a group of athletes coached by Aden, sources close to the investigation told AFP.
A physiotherapist was also arrested and the two were charged with having administered athletes with banned substances, sources added.
Six doctors from world athletics' governing body, the IAAF, and officials from the Spanish Anti-Doping Agency were also present to carry out doping controls on some athletes.
Aden has a proven track record of nurturing some of the best middle-distance runners.
Aside from Dibaba, he has coached two-time world indoor 800m champion Abubaker Kaki Khamis and 2008 Beijing Olympics 800m silver medallist Ismael Ahmed Ismael, both from Sudan, 2012 London Olympics 1500m champion Taoufik Makhloufi of Algeria, 2012 world junior 1500m champion Hamza Driouch, a Moroccan-born Qatari, and 2013 world indoor 1500m champion Ayanleh Souleiman of Djibouti.
But Aden has also courted controversy, with Driouch handed a two-year ban for inconsistencies in his biological passport and French runner Laila Traby also being banned after testing positive for blood-boosting EPO.
Driouch at first claimed Aden had doped him, but later retracted his allegations.
Dibaba, sister of running legend Tirunesh, has seen her pre-Rio Olympic plans hampered by a toe injury that has recently ruled her out of meetings in Eugene, Oslo and Stockholm.
The 25-year-old world record holder is the big favourite to strike gold in the women's 1500m at the August 5-21 Rio Games.