The release of a once-secret list of Obama-era officials who sought to unmask what turned out to be the identity of Michael Flynn in intelligence reports has fueled demands from Republican lawmakers for more information about the extent of the practice — specifically during the presidential transition period.

Those demands reflect suspicions by President Trump allies that the list showing requests to “unmask” Flynn only represents the tip of the iceberg. However, sorting out which requests may or may not have been in any way improper is sure to be a challenging task.

LIST OF OFFICIALS WHO SOUGHT TO 'UNMASK' FLYNN RELEASED: BIDEN, COMEY, OBAMA CHIEF OF STAFF AMONG THEM

Data obtained by Fox News shows that thousands of unmasking requests have been fulfilled every year across both the Obama and Trump administrations, reflecting the often routine nature of these requests in intelligence work. The practice is regarded as an important national security tool, a view reflected by these numbers.

The Office of the Director of National Intelligence reported that in 2019, the Trump administration had 10,012 unmasking requests fulfilled; 16,721 requests fulfilled in 2018, and 9,529 requests fulfilled in 2017.

During 2016, the last year of the Obama administration, there were 9,217 unmasking requests fulfilled.

The numbers reflect how frequently national security and intelligence officials use this tool in their work. Unmasking occurs after U.S. citizens' conversations are incidentally picked up in conversations with foreign officials who are being monitored by the intelligence community. The U.S. citizens' identities are supposed to be protected if their participation is incidental and no wrongdoing is suspected. However, officials can determine the U.S. citizens' names through a process that is supposed to safeguard their rights. In the typical process, when officials are requesting the unmasking of an American, they do not necessarily know the identity of the person in advance.

But Republicans are highly suspicious of the number of unmasking requests made concerning Flynn, and have questioned whether other Trump associates were singled out. The list released Wednesday only covers the period of time between Election Day 2016 and the inauguration.

TRUMP WEIGHS IN ON UNMASKING: 'GREATEST POLITICAL CRIME IN THE HISTORY OF OUR COUNTRY'

“The unmasking of General Flynn by the Obama Administration regarding conversations during the presidential transition are deeply troubling and smell of politics, not national security,” Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., said in a statement Wednesday. “The unmasking of General Flynn by the Obama Administration regarding conversations during the presidential transition are deeply troubling and smell of politics, not national security.”

He added: “I specifically want to know how many unmasking requests were made, if any, beyond General Flynn regarding members of the Trump campaign team, family, or associates.”

Meanwhile, Sen. Rand Paul, R-Ky., said he was “greatly disturbed” by the list of unmasking requests concerning Flynn and said that the newly released information “gives a window into possible abuses of power motivated by political decisions, by individuals at the highest levels of government, including Vice President Joe Biden.”

The heightened speculation came after Sens. Chuck Grassley, R-Iowa, and Ron Johnson, R-Wis., made public a list of Obama officials who purportedly requested to “unmask” the identity of Flynn, who at the time was Trump’s incoming national security adviser.

The list was declassified by Acting Director of National Intelligence Richard Grenell and sent to Grassley and Johnson.

The roster featured top-ranking figures including then-Vice President Joe Biden, then-FBI Director James Comey, then-CIA Director John Brennan, then-Director of National Intelligence James Clapper, and Obama’s then-chief of staff Denis McDonough.

READ THE DOCUMENTS

As Fox News previously reported, Grenell already made the decision to declassify information about Obama administration officials who were involved in the “unmasking” of Flynn — whose calls with the former Russian ambassador during the presidential transition were picked up in surveillance and later leaked. His case has returned to the national spotlight after the DOJ moved to dismiss charges against him of lying to the FBI about those conversations, despite a guilty plea that he later sought to withdraw.

The declassified list specifically showed officials who “may have received Lt. Gen Flynn’s identity in response to a request processed between 8 November 2016 and 31 January 2017 to unmask an identity that had been generically referred to in an NSA foreign intelligence report,” the document said.

“Each individual was an authorized recipient of the original report and the unmasking was approved through NSA’s standard process, which includes a review of the justification for the request,” the document said. “Only certain personnel are authorized to submit unmasking requests into the NSA system. In this case, 16 authorized individuals requested unmasking for [REDACTED] different NSA intelligence reports for select identified principals.”

The document added: “While the principals are identified below, we cannot confirm they saw the unmasked information. This response does not include any requests outside of the specified time-frame.”

The list revealed that then-U.S. Ambassador to the United Nations Samantha Power made unmasking requests seven times between Nov. 30, 2016 and Jan. 11, 2017. The list revealed that Clapper made three requests from Dec. 2, 2016 through Jan. 7, 2017; and that Brennan made two requests, one on Dec. 14 and one on Dec. 15, 2016. Comey also made a request on Dec. 15, 2016. On Jan. 5, 2017, McDonough made one request, and on Jan. 12, 2017, Biden made one request.

In a statement on Wednesday, Andrew Bates, Biden's director of rapid response, downplayed the latest Flynn revelations.

"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,” Bates said. “What's more, it's telling that these documents were selectively leaked by Republicans abusing their congressional powers to act as arms of the Trump campaign after having them provided by a partisan official installed for this very purpose.”

GRENELL WON'T TESTIFY, DESPITE RAND PAUL REQUEST IN WAKE OF FLYNN UNMASKING REVELATIONS

Officials in the Obama administration have acknowledged that they moved to unmask some Americans in intelligence reports, but insisted that their reasons were legitimate. Those officials include Power as well as former national security adviser Susan Rice and former deputy national security adviser Benjamin Rhodes, who were not on the list released Wednesday.

Meanwhile, President Trump blasted the Obama administration over the revelations and claimed that "it was the greatest political crime in the history of our country."

"If I were a Democrat instead of a Republican, I think everybody would have been in jail a long time ago, and I'm talking with 50-year sentences. It is a disgrace what's happened. This is the greatest political scam, hoax in the history of our country," Trump said during an exclusive interview Thursday with Fox Business' Maria Bartiromo. "People should be going to jail for this stuff and hopefully, a lot of people are going to have to pay."

He added: "No other president should have to go through, and I'll tell you, Gen. Flynn and others are heroes, heroes, because what's happened to them. They weren't after Gen. Flynn. They wanted him to lie about me. Make up a story."

Fox News' David Spunt and Madeleine Rivera contributed to this report.