Mr Giuliani, the 76-year-old attorney and former mayor of New York, was the target of two raids on an apartment and office by the FBI in recent days.
Following the twin raids and FBI investigations, allies of the former president and former members of Mr Trump’s inner circle are reportedly increasingly concerned about further raids.
An adviser to the former president told CNN: "This was a show of force that sent a strong message to a lot of people in Trump's world that other things may be coming down the pipeline”.
Two other Trump allies also referenced concerns of other federal investigations on former members of his inner circle, the broadcaster said.
Mr Giuliani, a lawyer for Mr Trump, denies any allegations of wrongdoing, including related to his efforts to find information on the Biden family in Ukraine in the run up to the 2020 election in Ukraine. It is believed he is being investigated for allegedly failing to register as a foreign agent while representing Ukrainian interests in the US.
Federal prosecutors are also investigating whether or not Mr Giuliani was responsible for the firing of Marie Yovanovitch as US ambassador to Ukraine by Mr Trump, according to the New York Times.
Ms Yovanovitch subsequently gave evidence to Congress during Mr Trump’s first impeachment in 2019. It was claimed she had been forced out for frustrating Mr Giuliani’s efforts to dig up political dirt on the Bidens in the former Soviet state.
"Even the most loyal people have their breaking point," said a person close to Mr Trump, adding that if Mr Giuliani was to stop being supportive of the former president, "[it] wouldn't shock me at all."
"I think we've seen some more surprising instances of things like that happening, especially with Michael Cohen," the person close to Mr Trump said, referring to the former president’s one-time lawyer and fixer who turned against him.
The raids on Mr Giuliani follow the departure of Mr Trump from office in the wake of falsely alleging the 2020 election was stolen by Mr Biden, and of the presidential pardoning of a number of former aides.
Mr Cohen was not pardoned – he had been convicted of lying to Congress during the Mueller investigation into the Trump campaign’s ties to Russia.
Andrew Giuliani, the son of Mr Giuliani, told CNN on Thursday that is impossible to say if his father would turn on Mr Trump if he is charged with any wrongdoing.
“No! I mean, he has – there is – I don’t really know what to, how to respond to this because it’s a theoretical,” said the younger Giuliani.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies