The Federal Bureau of Investigation is helping the United States Anti-Doping Agency's long-running probe into Sir Mo Farah's coach, Alberto Salazar.
Farah joined Salazar's Nike Oregon Project (NOP) stable in 2011 and the pair have enjoyed huge success together, with the British star winning four Olympic gold medals and five world titles.
Their success has not come without scrutiny, though, and USADA has been investigating Salazar's methods for at least four years.
It is understood the FBI has recently taken an interest in that investigation, a development first reported by London's Daily Telegraph on Wednesday.
Neither the FBI nor USADA, however, would officially confirm the news, and Nike, which bankrolls NOP, said it "does not comment on rumour or speculation".
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A representative for Farah declined to comment, as did UK Anti-Doping (UKAD), and Salazar has not replied to requests for a statement.
World Anti-Doping Agency director general Olivier Niggli was also reluctant to comment on an ongoing investigation but said he backed the principle of law enforcement getting involved in complex anti-doping cases.
Citing evidence obtained from Italian police in the Lance Armstrong case, Niggli said national law enforcement bodies can conduct far more thorough investigations than national anti-doping agencies, as they have greater powers to access documents and financial records and can compel witnesses to cooperate.
"Clearly, there needs to be a breach of the law first, and that is why it is so important for governments to enact laws related to trafficking drugs and so on, and to enforce them," he said.
News of the FBI's involvement comes only days after it was reported that USADA has asked its British counterpart UKAD for access to Farah's anti-doping samples in order to re-test them.
Press Association Sport understands that USADA believes testing technology has already advanced far enough to reanalyse a batch of the samples.
