The Justice Department shed new light Monday on its 2019 decision-making process surrounding whether then-President Donald Trump obstructed justice, telling a court he couldn't have been indicted while serving as president no matter what special counsel Robert Mueller found in the course of his 22-month Russia investigation.

The development is the result of a legal fight sparked earlier this month when a U.S. District Court judge ordered the Justice Department to release an internal memo analyzing whether Mr. Trump obstructed justice. The Department said it would appeal to keep its legal reasoning in the matter under wraps because it constituted confidential legal advice from the department’s lawyers to then-Attorney General William Barr.

The Biden administration sided with its predecessor by saying it intends to appeal the case. The Justice Department didn’t immediately respond to a request for comment.

In the late Monday filing, the Justice Department said Mr. Barr never considered bringing charges against Mr. Trump due to a longstanding and controversial department policy that a sitting president can’t be indicted.

“Commencing an actual prosecution of the president was not an option the attorney general was considering,” the department said in its filing.