Mr Mueller has hired lawyers experienced in handling money-laundering cases and the Mafia in his pursuit of witnesses and evidence, Axios reported.
The US President has previously warned Mr Mueller not to look into his family's finances beyond the scope of his investigation, saying it would be crossing a red line.
Members of the GOP are concerned the new hires suggest Mr Mueller will begin looking into Mr Trump's financial dealings, including a proposal made during his election campaign to build a Trump Tower in Moscow.
It comes after Mr Trump's former advisor Steve Bannon told 60 Minutes the President's firing of FBI director James Comey may have been the worst mistake in "modern political history."
The website reports close White House allies believe Mr Mueller could use events concerning the firing to make an obstruction of justice case against Mr Trump.
The firing of Mr Comey led to the appointment of Mr Mueller to oversee the investigation into possible collusion between the Trump campaign and Russia.
An "angry, meandering" draft letter justifying the firing of Mr Comey, which was never released but Mr Mueller has acquired, could be used as evidence, Axios also said.
The Trump administration has reportedly already started handing over documents to Mr Mueller's team after the special prosecutor requested the White House preserve a number of documents relating to various members of Mr Trump's campaign team.
He also asked the White House to preserve evidence relating to former general Michael Flynn, who was forced to resign as Mr Trump's national security adviser, and a June 2016 meeting between a Russian lawyer linked to the Kremlin and the President's eldest son, son-in-law and campaign manager.