By Aruna Viswanatha, Dustin Volz, Wall Street Journal–

The National Security Agency received and approved requests on behalf of more than three dozen Obama administration officials, including then-Vice President Joe Biden, to “unmask” a U.S. citizen mentioned in classified foreign intelligence reports, revealing the identity of Michael Flynn.

The requests were made between President Trump’s November 2016 election and inauguration in January 2017, according to a memo declassified by Mr. Trump’s new acting director of national intelligence and made public Wednesday by two Republican senators. Mr. Flynn resigned just weeks into his tenure as Mr. Trump’s first national security adviser over his conflicting statements about his contacts with Russian officials before the inauguration.

In addition to Mr. Biden, the presumptive Democratic presidential candidate against Mr. Trump in November, the officials listed as making the request included then-U.S. Ambassador to the United Nations Samantha Power, then-Central Intelligence Agency Director John Brennan and President Obama’s chief of staff Denis McDonough, as well as a range of other senior and lower-ranking officials across several government departments.

Procedures for unmasking are designed under the law that governs the covert surveillance of foreigners to allow officials to better understand classified intelligence reports where U.S. citizens’ names are typically redacted to protect their identity.

All those who requested the reports were authorized to receive them, according to information contained in a memo from NSA Director Paul Nakasone dated May 4. The unmasking requests were approved through the NSA’s regular process, which includes a review of the justification for the request, the memo said. The release didn’t make clear what specific conversations were unmasked or identify the foreigners with whom Mr. Flynn had spoken.

The release is expected to fuel a partisan battle that has been waged since the 2016 election. Republican supporters of Mr. Flynn, an early backer of Mr. Trump’s campaign, say his career as national security adviser was ruined through leaks of classified information about his communication with the Russian ambassador in Washington at the end of 2016, when the Obama administration was imposing sanctions on Moscow for interfering in the U.S. election.

Many Democrats and current and former intelligence officials view the scrutiny of foreign intercepts during the presidential transition—and of Mr. Flynn’s role in those months—as warranted, given that intelligence officials had already issued warnings about Russia’s election interference and possible ties between Mr. Trump’s 2016 campaign and Moscow.

On Wednesday, Mr. Trump’s 2020 campaign accused Mr. Biden of improperly using the unmasking process to target Mr. Flynn. “Americans have a right to know the depth of Biden’s involvement in the setup of Gen. Flynn to further the Russia collusion hoax,” Brad Parscale, the campaign manager of Mr. Trump’s re-election campaign, said.

Andrew Bates, a spokesman for Mr. Biden’s 2020 presidential campaign, said: “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.”

Ms. Power didn’t respond to a request for comment. Through a spokesman, Mr. Brennan declined to comment. Mr. McDonough didn’t respond immediately to a request for comment.

The officials named in the memo, including Mr. Biden, wouldn’t have known Mr. Flynn’s identity until after receiving the unmasked reports from the NSA. Mr. Biden’s request to see an unmasked intercept occurred on Jan. 12, 2017, eight days before he left office.

Unmasking U.S. identities in intelligence reports occurs thousands of times annually, according to statistics maintained by the Office of the Director of National Intelligence.

“You can’t do your job without it,” said Michael Morell, the former acting director of the Central Intelligence Agency during the Obama administration and host of the Intelligence Matters podcast, who added that he used to make unmasking requests several times a month.

Mr. Morell said that Obama administration officials in several agencies would have had justified national security reasons to unmask Mr. Flynn’s conversations with the Russian ambassador, including ensuring that it wasn’t someone from within the administration who was undermining Mr. Obama’s decision on the sanctions.

The memo was released by GOP Sens. Ron Johnson (R., Wis.), chairman of the Senate Homeland Security and Governmental Affairs Committee, and Chuck Grassley (R., Iowa), chairman of the Senate Finance Committee. The information was declassified last week by Richard Grenell, Mr. Trump’s acting director of national intelligence.

Mr. Flynn resigned as national security adviser within weeks of Mr. Trump’s 2017 inauguration after reports emerged he had misled Vice President Mike Pence and others about phone calls Mr. Flynn had with Russia’s ambassador, Sergey Kislyak, which were first reported in an Opinion column in the Washington Post dated Jan. 12, 2017. His lawyer didn’t respond to a request for comment.

The memo doesn’t explain the reasons for the unmasking requests, most of which occurred two weeks before the phone call in late December between Mr. Flynn and Russia’s ambassador, which later became the reason for his resignation. Intelligence about Mr. Flynn’s call with Mr. Kislyak was provided by the FBI, not the NSA, and the FBI chose to unmask Mr. Flynn’s name proactively as it briefed a small group of senior officials, including Messrs. Obama and Biden, according to congressional testimony from former FBI Director James Comey.

Current and former intelligence officials said the memo didn’t demonstrate that Mr. Flynn—who would have been identified, for example, as “U.S. Person 1” in the initial intelligence reports before being unmasked—was wrongfully surveilled.

“What this indicates is he was in communication with people who were foreign intelligence targets,” said April Doss, a former National Security Agency lawyer and a partner at Saul Ewing Arnstein & Lehr LLP. Ms. Doss, who worked under Democratic and Republican administrations, said presidential transitions are sensitive counterintelligence periods because foreign adversaries are seeking to gain any possible influence with individuals who may hold power in an incoming administration.

Often incoming officials don’t realize the true identity or motives of a foreigner they are communicating with, and they or the president may need to be warned about the counterintelligence risks. In Mr. Flynn’s case, Mr. Obama warned Mr. Trump against hiring Mr. Flynn as his national security adviser, The Wall Street Journal has previously reported. In 2014, Mr. Obama, a Democrat, fired the then-lieutenant general from his senior Pentagon post as head of the Defense Intelligence Agency.

“The list of individuals who requested unmasking doesn’t necessarily evince a political motivation, but clearly indicates significant concerns across the government about the communications taking place between the incoming administration and foreign officials,” said Jamil Jaffer, a former White House lawyer under President George W. Bush and the executive director of the National Security Institute at George Mason University Law School.

Still, recent revelations about the FBI’s handling of surveillance applications for former Trump campaign adviser Carter Page, as well as how FBI agents conducted their early 2017 interview with Mr. Flynn, did raise serious and justifiable concern about the individual actions of some officials, Mr. Jaffer said.

In addition to political figures like Mr. Biden and Ms. Power, unmasking requests also were made by lower-level Treasury Department officials and U.S. ambassadors, as well as Mr. Comey, who didn’t respond to a request for comment.

Mr. Flynn in 2017 pleaded guilty to lying to the Federal Bureau of Investigation about his calls with the Russian ambassador, admitting he had misled the agents about whether he had asked Russia to moderate its response to sanctions levied by Mr. Obama. As part of his plea agreement, Mr. Flynn admitted to lying about other calls with foreign diplomats, including about a U.N. Security Council resolution vote in late December 2016.

Mr. Flynn cooperated with special counsel Robert Mueller’s investigation into Russian interference in the 2016 election.

Earlier this year, Mr. Flynn asked to withdraw his previous guilty plea for lying to the FBI, and argued he had been set up by the FBI and entrapped by agents.

Last week, the Justice Department filed a motion to dismiss the case, arguing that former FBI officials improperly sought to interview Mr. Flynn about his Russian interactions, both because they had been about to close an investigation into him and had deliberately failed to coordinate the interview with Justice Department officials.

The judge overseeing Mr. Flynn’s case, U.S. District Judge Emmet Sullivan, on Wednesday appointed a retired judge, John Gleeson, to present arguments in opposition to the government’s motion to dismiss the case against Mr. Flynn. In the order, Judge Sullivan told Mr. Gleeson to address whether the court should consider holding Mr. Flynn in criminal contempt for perjury.

In his memo, Mr. Nakasone said 16 authorized individuals had made unmasking requests on behalf of more than three dozen senior officials. The NSA wasn’t able to confirm whether all of the officials saw the unmasked information, the memo said.

The earliest request cited in the memo was from Ms. Power, on Nov. 30, 2016. Many of the requests came between Dec. 14 and Dec. 16.

Write to Aruna Viswanatha at Aruna.Viswanatha@wsj.com and Dustin Volz at dustin.volz@wsj.com