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.
Press Association Sport understands America's national law enforcement agency, the FBI, has recently taken an interest in that investigation, a development first reported by the 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".
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, in London for the Tackling Doping in Sport conference, 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.
Niggli said: "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."
News of the FBI's involvement in the Salazar investigation comes only days after it was reported by the Daily Mail and Sunday Times that USADA has asked its British counterpart UKAD for access to Farah's anti-doping samples in order to re-test them.
It is understood that USADA believes testing technology has already advanced far enough to reanalyse a batch of the samples it has stored from NOP athletes without waiting until later in the 10-year statute of limitation window.
As well as repeatedly testing NOP athletes such as American duo Matthew Centrowitz and Galen Rupp, USADA has retested their samples and Farah's and they have all come back negative.
UKAD, however, has not yet complied with USADA's request, as it has concerns about the risk of the samples becoming degraded, although talks between the agencies, who work closely together, are believed to be ongoing.
For his part, Farah has said he is happy to be drug-tested "any time, anywhere" and for any of his historical samples to be reanalysed.
Farah has remained loyal to Salazar ever since the BBC and US website ProPublica reported allegations in 2015 about his use of testosterone supplements and thyroid medication.
USADA was already investigating Salazar at this point but its inquiries had appeared to have gone cold last year while Centrowitz, Farah and Rupp were all winning medals at the Rio Olympics.
However, it has recently emerged USADA's investigation remains alive, with a particular focus on NOP's Houston-based doctor Jeffrey Brown.
A 269-page dossier written by the anti-doping agency to lay out its case against Brown, who it is pursuing via the Texas Medical Board, was stolen by Russian computer hackers the Fancy Bears and leaked to several European newspapers.
This report, which is now a year old, made a number of serious allegations against both Brown and Salazar but USADA has not yet followed up by opening anti-doping rule violation cases against either, and the Texas Medical Board is still conducting its own investigation into the doctor.