The federal prosecutor tasked with probing the "unmasking" of President Donald Trump's transition team by Obama administration officials has concluded his investigation without finding evidence of any substantive wrongdoing, the Washington Post reported Tuesday, citing unnamed sources familiar with the matter.

What are the details?

Attorney General William Barr assigned U.S. Attorney John Bash in May to look into whether Obama officials had improperly "unmasked" Trump team members — that is, requested their identities be revealed after they were incidentally picked up in the federal government's surveillance of foreign officials.

The matter, now reportedly closed without charges or a report, was already being investigated as a part of U.S. Attorney John Durham's probe into the origins of the Trump-Russia investigation, but Barr believed it to be worthy of further consideration.

In a statement announcing the investigation in May, Justice Department spokeswoman Kerri Kupec said: "Unmasking inherently isn't wrong, but certainly, the frequency, the motivation and the reasoning behind unmasking can be problematic. And 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."

At the time, newly unsealed documents purportedly showed that several high-ranking Obama officials had submitted unmasking requests for former Trump national security adviser Gen. Michael Flynn — including then-Vice President Joe Biden, then-FBI Director James Comey, and then-Director of National Intelligence James Clapper.

News of the probe's conclusion came as Bash stepped down from his post as U.S. attorney in the Western District of Texas last week to accept a job in the private sector.

At the time, Kupec made a statement, saying, "Without commenting on any specific investigation, any matters that John Bash was overseeing will be assumed by Gregg Sofer." The Washington Post reported that when asked whether Bash's resignation was related to the unmasking probe, Kupec responded, "No, that is not my understanding."

What else?

According to NBC News, Brennan and another unnamed key figure in the unmasking controversy reported that they were not interviewed by Bash or Justice Department investigators as a part of the probe.

"I was never interviewed by anyone in the Department of Justice about unmasking, which was a politically motivated probe initiated by William Barr to please Donald Trump," Brennan told the news outlet.

The New York Times, in its report, added that Bash's findings were handed over to Durham's team, according to Justice Department officials. It is important to note that the Post, which first reported the news about Bash's conclusions, "was unable to review the full results of what Bash found."

It was not immediately clear why a public report about Bash's findings has not been made available by the Justice Department. The Post noted that while not reporting the findings of probes that conclude without criminal charges is not unusual, a public report was considered more likely given the public nature of the investigation.

Why does it matter?

The conclusion of Bash's report without any charges is likely to rankle President Trump as well as Republicans on Capitol Hill who had hoped the Justice Department would expose what they believed to be wrongdoing by the Obama administration in unmasking individuals before and after the 2016 election.

"The big story is the 'unmasking and surveillance' of people that took place during the Obama Administration," Trump tweeted early on in his presidency.

Sen. Rand Paul (R-Ky.) declared in May that Biden had been "caught red-handed" after the list of Obama officials who had requested Flynn's unmasking was released.

"This is Vice President Biden using the spying powers of the United States to go after a political opponent. He's caught red-handed here. Vice President Biden is caught red-handed eavesdropping on a political opponent's phone calls," Paul said.