President Donald Trump has tapped a longtime legal adviser to serve as his private attorney while a special council investigates whether his campaign worked with Russia in last year's election, a source familiar with the decision told Reuters Tuesday.

Fox Business News and ABC first reported that Trump hired Marc Kasowitz, a New York-based trial lawyer known as a tenacious litigator, to represent him in a Justice Department investigation headed by former Federal Bureau of Investigation Director Robert Mueller.

The appointment of a private attorney may indicate that Trump is seriously considering the impact the federal investigation could have on him personally and that he may wish to protect himself should others in his administration turn against him.

Kasowitz has represented Trump for more than 15 years. During last year's presidential campaign, Kasowitz threatened to sue the New York Times if it did not retract a story about women who accused Trump of allegedly touching them inappropriately. The Times did not retract the story despite lack of proof.

He also assisted in the defense of fraud claims against Trump University. After the election, Trump settled the claims for $25 million. Trump has looked to Kasowitz's firm to fill positions in his administration. He has described former Senator Joseph Lieberman, a senior counsel at the firm, as a top candidate to serve as FBI director and is considering Edward McNally, a defense lawyer at the firm, as the next US attorney in Manhattan. A former partner at the firm, David Friedman, was chosen by Trump as US ambassador to Israel.

Kasowitz also represents OJSC Sberbank of Russia, the country's largest bank, which is being accused in a US federal court of conspiring with granite company executives and others to raid the assets of a competitor.

The outside counsel would be separate from the White House Counsel's Office, led by Donald McGahn.

Mueller was appointed as special counsel by the Justice Department last week to investigate the Trump campaign's alleged ties to Russia. Several congressional committees and the FBI are also investigating the matter.

President Trump and Russia have denied all collusion allegations. Controversy has engulfed Trump since he fired FBI Director James Comey two weeks ago as Comey oversaw an investigation into possible collusion between his presidential campaign and Russia. The reasons for the dismissal of Comey, however, do not appear to be related to the actual investigation as it is ongoing.

Proof of the supposed collusion between the Trump campaign and the Russian government has also failed to surface, despite ongoing claims of a conspiracy; thus for now it remains just a theory.