The witnesses have cooperated with the demand, the report said. Yet it remains unclear whether the FBI found any conversations that could be used as evidence against US President Donald Trump.
The report comes just days after Mueller on Monday filed a motion accusing Trump's former campaign manager Paul Manafort of attempting to tamper with witnesses. Muller's filing alleged that Manafort used encrypted messages to contact witnesses by phone about his fraud and tax evasion case.
The charges came as part of Mueller's probe into allegations that Russia interfered in the 2016 US presidential election, as well as accusations that Trump's campaign team colluded with Moscow to affect the outcome of the vote. The White House and the Kremlin have fully denied the accusations.