Acting US Director of National Intelligence Richard Grenell sent two Republican lawmakers a list of senior officials in former President Barack Obama's administration, who were alleged to have been involved in "unmasking" former National Security Adviser (NSA) Michael Flynn from an intelligence report.
Unmasking is a term that the US intelligence community uses to refer to revealing the identity of someone in a monitored communication, reports Xinua news agency.
On Wednesday, Grenell, in a letter to Republican Senators Ron Johnson and Chuck Grassley, said that he declassified the document last week and that he was providing it to them for "situational awareness".
A memo signed by General Paul Nakasone, director of the National Security Agency, said the list includes identities of any officials who submitted requests to the agency at any point between November 8, 2016 and January 31, 2017 to unmask the identity of Flynn, who was also a senior advisor to Donald Trump's 2016 presidential campaign.
Then Vice President Joe Biden, former White House chief of staff Denis McDonough, former FBI Director James Comey, and former CIA Director John Brennan are among those on the list.
They "may have received" Flynn's identity in response to a request to unmask an identity "that had been generically referred to in an National Security Agency foreign intelligence report", the declassified document reads.
"Each individual was an authorized recipient of the original report and the unmasking was approved through National Security Agency's standard process."
In response, Johnson, chairman of the Senate Homeland Security and Governmental Affairs Committee, and Grassley, chairman of the Senate Finance Committee, said in a joint statement that they "wholeheartedly agree that transparency was needed now more than ever".
"One of the most significant unanswered questions about what occurred during the 2016 election is how many Americans were 'unmasked,' at whose request, and for what purpose," they wrote.
Republican Senator Rand Paul, an ally of Trump, told reporters on Wednesday that he wants a hearing on this matter.
The intelligence community frequently monitors the correspondence of foreign actors non-national individuals or groups that hold political influence including businessmen and ambassadors operating on US soil, according to Business Insider.
Unmasking occurs when intelligence officials need more context about the surveillance on a foreign actor to make sense of the information they have received.
In Flynn's case, the National Security Agency was monitoring then Russian ambassador Sergey Kislyak.
Flynn admitted in December 2017 that he lied to the FBI about his discussions, including of US sanctions against Russia, with Kislyak.
Criminal charges against Flynn became part of former special counsel Robert Mueller's inquiry into Moscow's alleged meddling to help Trump win in 2016, which the President has repeatedly called a "witch hunt" and "hoax".
In a court filing last week, the Department of Justice said it was abandoning the prosecution of Flynn "after a considered review of all the facts and circumstances of this case, including newly discovered and disclosed information", a move that has triggered a political firestorm.
( With inputs from IANS )