President Trump’s top intelligence adviser has given the Justice Department the names of Obama administration officials who “unmasked” Michael Flynn following his conversations with the Russian ambassador to the United States in 2016, according to U.S. officials.

That action, which has been a cornerstone of Trump’s long-standing allegations of criminality by his predecessor, identified Flynn, then the national security adviser-designate, as the person urging Russia not to respond to punitive sanctions that the Obama administration had imposed after the Kremlin’s interference in the presidential election.

Unmasking is a routine practice used to identify a U.S. person who is anonymously referred to in an intelligence document — in this case the intercepted conversations of Sergey Kislyak, then the Russian ambassador, who was a target of U.S. surveillance. Current and former officials said unmasking can be a vital tool for identifying potential spies or terrorists.

Richard Grenell, the acting director of national intelligence, made the decision to declassify the list of officials involved, an action first reported by ABC News. Grenell provided the names to the Justice Department the day after it filed a motion to drop charges against Flynn, who pleaded guilty to lying to the FBI about his conversations with Kislyak. Flynn was also fired as national security adviser for lying to Vice President Pence about those communications, the White House said at the time.

Flynn’s communications with Kislyak were scrutinized as part of the FBI’s investigation of Russian election interference and possible ties between the Trump campaign and Russia. Last year, Attorney General William P. Barr appointed a career federal prosecutor, John H. Durham, to investigate the probe’s origins. Separately, the Justice Department inspector general found that the investigation was properly started and not influenced by political bias, but also found broad and “serious performance failures” requiring major changes.

A Justice Department official said the department had “been reviewing unmasking as part of our broader review of 2016 and 2017.” That would seem to refer to the investigation being conducted by Durham, and perhaps a related inquiry by U.S. Attorney Jeff Jensen into high-profile cases in the D.C. U.S. attorney’s office. The official, who spoke on the condition of anonymity to discuss a sensitive matter, declined to be more specific.

Grenell’s office “delivered information related to unmasking to the department, and to the extent it’s relevant to any investigation, the department will take a look at it,” the official said, adding that the Justice Department “does not intend to release the list” of those who directed unmaskings.

It was not clear if Grenell would release the names on his own. A spokesperson for the Office of the Director of National Intelligence referred questions on the matter to the Justice Department.

Releasing the names would be an unprecedented action and risk turning powerful authorities to declassify intelligence toward political ends, current and former intelligence officials said.

“Unmasking is common — literally hundreds of times a year across multiple administrations,” said Michael Morell, the former deputy director of the CIA and host of the “Intelligence Matters” podcast.

“In general, senior officials make the requests when necessary to understand the underlying intelligence,” Morell said. “I myself did it several times a month. You can’t do your job without it.”

Current and former officials defended the decision to unmask Flynn as vital to understanding if the Trump campaign, to which Flynn was a senior adviser, was seeking to undermine the Obama administration’s foreign policy.

“This is politics corrupting intelligence,” said one former senior official, who like others spoke on the condition of anonymity because of fears of retaliation by the Trump administration.

The Trump administration has offered no evidence that the unmasking in Flynn’s case was improper or didn’t follow standard rules.

FBI agents interviewed Flynn about his communications with Kislyak in an effort to advance their broader Russia probe, though the Justice Department indicated in a court filing last week seeking to undo Flynn’s plea that it has since decided the interview should not have been conducted.

Legal analysts have lambasted that decision, which Jensen recommended and Barr ultimately made, asserting it seemed to be an example of the attorney general working to assist an associate of Trump.

A federal judge on Tuesday signaled he wouldn’t immediately acquiesce to the department’s request, indicating on the court’s docket that he would accept filings from independent groups and legal experts who want to weigh in on the matter. That could preface more aggressive steps that the judge could take, including — as many outside observers have called for — holding a hearing to consider what to do.

It wasn’t immediately clear why Grenell chose to declassify the names. Trump has already granted Barr extraordinary authority to declassify intelligence as part of the Russia probe investigation.

When Trump appointed Grenell to replace former DNI Daniel Coats, intelligence veterans worried about putting an outspoken political loyalist and defender of the president in charge of U.S. spy agencies. Grenell frequently tweets in support of Trump and his policies and attacks journalists he believes are treating the administration unfairly.

Grenell is also the U.S. ambassador to Germany.

The president has nominated Rep. John Ratcliffe (R-Tex.) as the permanent DNI. He is not expected to get a Senate vote for at least a few weeks, congressional aides have said.

The unmasking issue has been central to allegations by other presidential supporters that the Obama administration tried to harm Trump’s campaign and undermine his presidency.

Rep. Devin Nunes (Calif.), the ranking Republican on the House Intelligence Committee, has long described the use of the authority as abusive.

But in late 2016, when Nunes served as chairman, the committee engaged in the same practice, asking U.S. intelligence agencies to reveal the names of U.S. individuals or organizations contained in classified intelligence on Russia’s election interference, The Washington Post has previously reported.

This week, Trump again returned to his claims that the Obama administration had tried to undermine his campaign and his administration.

“Obamagate, it’s been going on for a long time,” Trump said at a news conference about the coronavirus on Monday, indicating that more information was forthcoming.

“It’s been going on from before I even got elected. And it’s a disgrace that it happened. And if you look at what’s gone on, and if you look at, now, all of this information that’s being released — and from what I understand, that’s only the beginning — some terrible things happened, and it should never be allowed to happen in our country again.”

This story has been updated to make clear that Michael Flynn was the national security adviser-designate at the time of his phone calls with the Russian ambassador in 2016.

Spencer S. Hsu contributed to this report.