That move ultimately led to the appointment of special counsel Robert Mueller, whose long-running investigation has been criticized bitterly by the president.
In his resignation letter, Sessions took one final jab at the president, writing: "Most importantly, in my time as Attorney General we have restored and upheld the rule of law-a glorious tradition that each of us has a responsibility to safeguard".
Sources familiar with the matter told the Post that Barr is a "really serious contender, and possibly the front-runner" for the position of attorney general.
Barr is now employed with the Kirkland & Ellis law firm in Washington, D.C. There, he still advises corporations with enforcement and regulatory issues, according to The Washington Post.
President Trump announced Friday his intention to nominate William Barr to be attorney general, according to the White House pool. He also said that there would be nothing wrong with then Attorney General Jeff Sessions calling for an investigation into both Clinton and James Comey in regards to Clinton's private email server.
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After leaving the Justice Department, Barr served as the general counsel and executive vice president of Verizon Communications but retired from the role in 2008.
Before that, he also served as deputy attorney general and assistant attorney general overseeing the Office of Legal Counsel.
A third source familiar with the process said if Trump offered the position to Barr, he would be willing to accept.
Barr also wrote previous year that the administration's decision to fire James B. Comey as FBI director was "quite understandable" because, in his view, Comey had usurped the power of the attorney general when he publicly announced his recommendation not to charge former Secretary of State Hillary Clinton during the investigation of her private email server. Respected by Republicans. Respected by Democrats.
Mr Barr has voiced disapproval over some aspects of the inquiry but his views on the investigation's legitimacy remain unknown.
Barr also at one point took issue with the political affiliations of those on Mueller's team, telling the Washington Post in July of 2017 that "prosecutors who make political contributions are identifying fairly strongly with a political party".
William Barr worked for the Central Intelligence Agency while going to law school at night.