U.S. 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.