Washington: President Donald Trump has tapped a longtime legal adviser to serve as his private attorney while a special counsel investigates whether his campaign worked with Russia in last year’s election, a source familiar with the decision said on 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.

The source said Kasowitz is expected to bring other lawyers on to work as a team to protect Trump.

Other candidates for the job, including Brendan Sullivan and Robert Giuffra, withdrew their names from consideration, the source said.

“He can be aggressive — he’s got that in him for sure,” said John Quinn of Quinn Emanuel Urquhart and Sullivan, who has worked with and opposite Kasowitz. “He also can be smooth as silk, respectful and deferential.” Kasowitz and the White House did not immediately respond to a request for comment from Reuters.

Kasowitz has represented Trump for more than 15 years but he is not known as a criminal defence lawyer. 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 touching them inappropriately. The Times did not retract the story.

He also assisted in the defence of fraud claims against Trump University, a series of real-estate seminars. 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 white-collar defence 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 possible ties to Russia. Several congressional committees and the FBI are also investigating the matter.

US intelligence agencies concluded in January that Moscow tried to sway the November vote in Trump’s favour. Russia has denied involvement, and Trump has denied any collusion between his campaign and Russia.

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.