American running coach Alberto Salazar is being investigated by the US Anti-Doping Agency to determine if anti-doping rules may have been violated.
A source has told Reuters that the agency is seeking documents and interviewing witnesses.
The source says 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 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's issued a lengthy and detailed denial of the allegations. USADA would not comment on whether an investigation was taking place.
USADA said in a statement that it 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.
The Cuban-born Salazar, who has worked as a consultant to British Athletics for two years, wrote in an open letter: "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."
Farah 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 and six-times national champion, plans to defend his title at the US world championships trials in Eugene, Oregon, this week.
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.
Among the allegations levelled at Salazar was that he had coached Rupp and other athletes on ways to manipulate therapeutic use exemptions (TUEs), in which athletes can use otherwise banned medications or treatments for medical reasons.
The IAAF said it "is very comfortable with the current policy on TUEs - but this does not mean that in future there may not be changes."