Deputy Attorney General Rod Rosenstein has officially handed in his resignation letter to the White House, a long-anticipated move. Rosenstein, who was expected to leave the Justice Department after the conclusion of special counsel Robert Mueller's investigation, will effectively resign May 11.

In his letter, Rosenstein thanked President Trump for appointing him to the job and "for the courtesy and humor you often display in our personal conversations."

"We enforce the law without fear or favor because credible evidence is not partisan, and truth is not determined by opinion polls," he wrote. "We ignore fleeting distractions and focus our attention on the things that matter, because a republic that endures is not governed by the news cycle."

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A White House official told CBS News Rosenstein's impending resignation is "not news," and pointed out the White House has already named a replacement for Rosenstein, Jeffrey Rosen. The official said they were expecting the letter this week.

Deputy U.S. Attorney General Rod Rosenstein seen July 13, 2018. Reuters

Rosenstein has weathered tumultuous times at the department in his more than two years in that role. It was his name on the letter describing the justifications for Mr. Trump to fire former FBI Director James Comey, and it was his decision to appoint Mueller, resulting in the special counsel's two-year investigation.

Attorney General William Barr acknowledged Rosenstein "navigated many challenging situations with strength, grace, and good humor."

"Rod has been an invaluable partner to me during my return to the Department, and I have relied heavily on his leadership and judgment over the past several months," Barr said in a statement. "I have appreciated the opportunity to work closely with him, and I wish him well in his future endeavors. The department and I will miss him."

Fin Gomez and Paula Reid contributed reporting.