WASHINGTON Before President Donald Trump forced former Attorney General Jeff Sessions to resign , the Justice Departments quasi-judicial Office of Legal Counsel told him he could appoint an official who had not been confirmed by the Senate, like acting Attorney General Matt Whitaker , to take Sessions place.

A 20-page OLC opinion authored by Assistant Attorney General Steven Engel, released by the Justice Department on Wednesday, states that OLC had previously advised that the President could designate a senior Department of Justice official, such as Mr. Whitaker, as Acting Attorney General.

The OLC opinion acknowledged that its rare for the acting attorney general to be an individual not confirmed by the Senate. The only example they found was from 1866, before the creation of the Justice Department. Nevertheless, the opinion concluded that Trumps appointment of Whitaker would be legitimate.

Following Attorney General Sessionss resignation, the President could have relied upon the Deputy Attorney General, the Solicitor General, or an Assistant Attorney General to serve as Acting Attorney General. But the availability of potential alternatives does not disable Congress from providing the President with discretion to designate other persons, Engel wrote. Nothing in the text of the Constitution or historical practice suggests that the President may turn to an official who has not been confirmed by the Senate if, but only if, there is no appropriate Senate-confirmed official available.

OLC, sometimes referred to as the presidents law firm, is a small unit within the Justice Department that plays an important role in the executive branch by offering internal legal reviews before the president or agency takes action. Its the office that signed off on President Barack Obamas use of drones against U.S. citizens, and the enhanced interrogation techniques during the George W. Bush era. Much of OLCs work is typically shielded from the public, but it does occasionally make its opinions public when it benefits the administration to do so such as when there are questions about whether the attorney general has been legitimately appointed.

A senior Justice Department official declined to say when, precisely, the White House asked OLC to opine on Trumps options should Sessions resign. The OLC opinion takes the position that it wouldnt have mattered whether Sessions was fired or resigned.

OLC cited dozens of examples of individuals taking control of top agency positions on an acting basis without being confirmed by the Senate, but could find only one example of an acting attorney general who had not been Senate-confirmed. J. Hubley Ashton, the opinion noted, was acting attorney general for six days in 1866 after the resignation of James Speed, who was appointed by Abraham Lincoln but resigned because he disagreed with President Andrew Johnson. Ashtons successor, Henry Stanbery, resigned as attorney general to represent Johnson during his impeachment trial.

Read the opinion below.

Ryan Reilly is HuffPosts senior justice reporter covering the Justice Department, federal law enforcement, criminal justice and legal affairs. Have a tip? Reach him at ryan.reilly@huffpost.com or on Signal at 202-527-9261.