For the past few days, Mr. Trump has publicly excoriated Mr. Barr and accused him of failing to deliver on charges against the president's perceived enemies.
"Unless Bill Barr indicts these people for crimes, the greatest political crime in the history of our country, then we're going to get little satisfaction unless I win," Mr. Trump told Fox Business last week.
He said he had asked the Justice Department to charge former President Barack Obama and former Vice President Joseph R. Biden Jr, Mr. Trump's Democratic opponent in the election next month, extraordinary requests by a sitting president to wield the power of federal law enforcement against political foes. The attorney general said this spring that neither was likely to even be investigated.
Updated Oct. 14, 2020, 5:36 p.m. ET
The president also said he was determined to find out why Mr. Durham was not ready to release a report. "He's got so much stuff," Mr. Trump said.
National security officials routinely make unmasking requests as they read and try to understand intelligence reports and other classified communications; for privacy reasons, names of Americans in the reports are blacked out, but officials can ask to see them to better understand the documents.
Such requests made by Obama administration officials during the presidential transition revealed conversations involving Mr. Flynn. For the past year, Mr. Trump and his allies have placed increasing pressure on the Justice Department to address those requests.
In May, Republicans released a list of names of Obama administration officials who had inquired in late 2016 and early 2017 about the identity of an American in National Security Agency intelligence reports that turned out to be Mr. Flynn, then Mr. Trump's incoming national security adviser. They included John O. Brennan, the C.I.A. director; Samantha Power, the ambassador to the United Nations; James R. Clapper Jr., the director of national intelligence; James B. Comey, the F.B.I. director; and Douglas E. Lute, the American ambassador to NATO.
But the list did not say what the intelligence reports were about, or whether they included surveillance of foreign officials talking about Mr. Flynn or of intelligence targets talking to him.
After the list was released, the Justice Department said that Mr. Barr had asked Mr. Bash to review whether the requests were irregular or improper, and then give his research to Mr. Durham.
"Unmasking inherently isn't wrong, but certainly the frequency, the motivation and the reasoning behind unmasking can be problematic," Ms. Kupec said in an interview with Sean Hannity of Fox News when she announced Mr. Bash's review.
"When you're looking at unmasking as part of a broader investigation -- like John Durham's investigation -- looking specifically at who was unmasking whom can add a lot to our understanding about motivation and big-picture events," Ms. Kupec said.
This is not the first time that the Justice Department under the Trump administration has pushed politically divisive work to U.S. attorneys far from the main department in Washington.
Former Attorney General Jeff Sessions asked John W. Huber, the United States attorney in Utah, to examine allegations by Mr. Trump and his allies about Hillary Clinton -- work that Mr. Durham also absorbed.
And federal prosecutors outside of Washington are accepting information about potential ties between Democrats and Ukraine from Rudolph W. Giuliani, the president's personal lawyer.