Three Republican senators on Wednesday made public a declassified list of U.S. officials, including former vice president Joe Biden, who made requests that would ultimately “unmask” Trump adviser Michael Flynn in intelligence documents in late 2016 and early 2017 — a common government practice but one that some conservatives have seized on to imply wrongdoing.

The list includes the names of more than three dozen former Obama administration officials. Among them are Biden, former White House chief of staff Denis McDonough, former FBI director James B. Comey, former CIA director John Brennan and former director of national intelligence James R. Clapper.

The list was declassified recently by President Trump’s top intelligence adviser, Richard Grenell, and given to the Justice Department. Grenell subsequently provided the list to Sens. Ron Johnson (Wis.), Charles E. Grassley (Iowa) and Rand Paul (Ky.), who made it public.

A cover letter indicates that those on the list submitted unmasking requests to the National Security Agency between Nov. 8, 2016, and Jan. 31, 2017. The requests would ultimately reveal Flynn’s identity, though a note on the list itself suggests that it was unknown whether all the officials actually “saw the unmasked information.” It was not immediately clear what documents or information the officials were examining when they made the requests.

Unmasking is a routine practice used to identify U.S. individuals who are referred to anonymously in an intelligence document, and it is meant to help government officials better understand what they are reading. But conservatives have long seized on Flynn’s unmasking to imply that he was treated unfairly by U.S. law enforcement and intelligence officials.

Trump told reporters after the document was released, “The unmasking is a massive thing,” and questioned Biden’s assertion to “Good Morning America” that he knew “nothing about those moves to investigate Michael Flynn.” Trump’s campaign manager, Brad Parscale, issued a similar statement, saying the Biden claim “just got more unbelievable.”

The Biden campaign rejected Trump’s characterization that the former vice president had acted inappropriately.

“These documents simply indicate the breadth and depth of concern across the American government — including among career officials — over intelligence reports of Michael Flynn’s attempts to undermine ongoing American national security policy through discussions with Russian officials or other foreign representatives,” Andrew Bates, a Biden campaign staffer, said in a statement. “Importantly, none of these individuals could have known Flynn’s identity beforehand. These documents have absolutely nothing to do with any FBI investigation and they confirm that all normal procedures were followed — any suggestion otherwise is a flat out lie.”

Kerri Kupec, a Justice Department spokeswoman, told Fox News on Tuesday that U.S. Attorney John Durham — who is examining the FBI’s investigation of the Trump campaign and possible connections with Russia in 2016 — was “already looking at this issue of unmasking” and would consider the recently declassified list “if it’s pertinent.”

“I can tell you that his team is working diligently to get to the bottom of what happened,” Kupec told host Martha MacCallum. “Because, Martha, what happened to candidate Trump and then President Trump was one of the greatest political injustices in American history and should never happen again.”

Conservatives seizing on unmasking is not new, despite the practice being common. Early in the Trump administration, the effort to scrutinize unmasking was pushed in part by Rep. Devin Nunes (R-Calif.), though the House Intelligence Committee he chaired also asked U.S. spy agencies to reveal the names of U.S. individuals or organizations contained in classified intelligence on Russia’s interference in the 2016 presidential election.

Some conservatives have connected unmasking to Flynn’s late December 2016 phone calls with then-Russian Ambassador Sergey Kislyak, who was the target of U.S. surveillance. The calls were later the subject of reports in The Washington Post. Flynn had urged Kislyak to refrain from retaliating for sanctions imposed by the Obama administration for interfering in the 2016 election, reassuring him that the new administration would seek to soften those measures.

Flynn pleaded guilty in 2017 to lying to the FBI about the phone calls, though the Justice Department last week moved to throw out the case after coming to the conclusion that agents did not have adequate basis to interview Flynn when they did so.

The list released Wednesday shows a flurry of unmasking requests in mid-December, weeks before the Flynn-Kislyak calls. Most of the U.S. requesters are not household names, but rather, Treasury, NATO and intelligence officials. What they were looking at, and why they were seeking to unmask someone, was not clear.

National security lawyer Mark Zaid suggested something might have happened around that time period to raise questions among government officials about Flynn.

“If you want to be transparent and fair, show us the document that led all these senior authorized government officials to request this information, that freaked them out all at the same time,” Zaid said.

The intelligence about the Flynn-Kislyak calls later in December was held by the FBI, not the NSA, and a summary describing them was circulated among senior Obama administration officials without Flynn’s identity being masked, according to former officials who spoke on the condition of anonymity to discuss a sensitive matter.

Knowing who was speaking to Kislyak was “absolutely essential to understanding the significance” of the conversation, a former senior U.S. official said.

That intelligence, a second former senior U.S. official said, “provided an explanation for the tepid response of [Russian President Vladimir] Putin to the sanctions that were imposed on the 29th of December because we all expected an immediate reciprocal response and we didn’t get it.”

​The unmasking controversy, said one of the former senior officials, “is once again a manufactured conflict for political reasons that has no basis in fact.”

In a statement, Johnson, who leads the Senate Homeland Security Committee, and Grassley, who heads its Finance Committee, said those on the list “should confirm whether they reviewed this information, why they asked for it and what they did with it, and answer many other questions that have been raised by recent revelations.” The pair had asked Grenell and Attorney General William P. Barr to turn over the list a day earlier.

“We are making this public because the American people have a right to know what happened,” the senators said, adding, “The records are one step forward in an important effort to get to the bottom of what the Obama administration did during the Russia investigation and to Lt. General Flynn.”

Paul released a portion of the list on Twitter and encouraged Senate colleagues to hold hearings on the matter.

A representative for Comey declined to comment. Clapper said unmasking is a regular and important part of a senior intelligence official’s duty.

“If U.S. persons are engaged with a valid foreign intelligence target, we should have at least a modicum of curiosity to try to understand what’s going on and does that pose a threat to national security,” he said. “To me, this was just part of my doing my duty to determine if there was a threat to the security of the United States.”

Though the president and his allies have vilified the process, unmasking has increased under the Trump administration. The NSA, which conducts legally authorized surveillance of communications overseas, unmasked 16,721 U.S. individuals in 2018, according to information compiled by the Office of the Director of National Intelligence. That was a more than 7,000-person increase over the previous year, when Trump took office.

The big jump may reflect an increase in the number of people or American businesses whose identities were revealed to determine if they were victims of computer hacking or some other crime perpetrated by foreign actors, officials have said.

In 2019, the NSA unmasked just over 10,000 U.S. individuals’ identities, a substantial decrease from the previous year, but still more than in the final year of the Obama administration, according to government records. The NSA unmasked 9,217 ​in the 12-month period between September 2015 and August 2016. The numbers are now compiled on a calendar-year basis.

Devlin Barrett contributed to this report.