American coach Alberto Salazar is being investigated by the U.S. Anti-Doping Agency (USADA) to determine if anti-doping rules may have been violated, a source told Reuters.
The agency is seeking documents and interviewing witnesses, the person, who is familiar with the investigation, said.
The probe has been ongoing and began before the BBC television programme Panorama in association with American website ProPublica made a series of allegations, the source said.
The allegations included that Salazar had given Olympic 10,000 metres silver medallist Galen Rupp the banned anabolic steroid testosterone.
Rupp is the training partner of British double Olympic champion Mo Farah, who has not been accused of any wrongdoing.
Salazar issued a lengthy and detailed denial of the allegations on Wednesday.
USADA would not comment on whether an investigation was taking place.
“USADA takes all reports of doping seriously and we aggressively follow up on all information we receive in order to fulfil our oath to protect clean athletes and the integrity of competition,” it said.
Consultant role
The Cuban-born Salazar, who has worked as a consultant to British Athletics for two years, wrote in an open letter published on Wednesday: “I believe in a clean sport and hard work and so do my athletes.
“I will never permit doping. At no time do we use science in violation of the WADA Code. We strictly adhere to competition and anti-doping rules at all times.
“I have not and will not condone any athlete I train using a banned substance and would never encourage any athlete to use a banned substance.”
Briton Farah, 32, won the 5,000 and 10,000 metres double at the 2012 London Olympics. American Rupp, 29, took the silver medal in the 10,000.
Rupp, the American record holder at 10,000 metres, plans to defend his title at the US world championships trials in Eugene, Oregon, today.
He has denied ever doping.
British Athletics said the content of Salazar’s statement would be referred to their performance oversight group for consideration in their ongoing internal review into the relationship between Salazar and Farah.