GOP Sens. Ron Johnson (Wis.) and Chuck Grassley (Iowa) are pressing the Trump administration to release the names of Obama-era officials who "unmasked" former national security adviser Michael Flynn.

The Republican senators sent a letter dated Tuesday to Attorney General William Barr and acting Director of National Intelligence Richard Grenell, urging them to release the names amid increased GOP scrutiny of the former Trump adviser's case.

“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,” the lawmakers wrote.

Johnson and Grassley chair the Senate Homeland Security and Governmental Affairs and the Finance committees, respectively. Their letter comes after several media outlets reported that Grenell declassified the names of Obama-era intelligence officials who sought to “unmask” Trump campaign associates such as Flynn who had been swept up in surveillance of foreign officials.

Identifying information of U.S. persons picked up during surveillance on a foreign individual must be hidden, or "masked," under U.S. law. Top intelligence officials can request that the identities be revealed.

Reports about Grenell's move to declassify the information came as the Justice Department moved to drop its charges against Flynn, who pleaded guilty in December 2017 to lying to federal agents about conversations he had with then-Russian Ambassador Sergey Kislyak.

The Justice Department’s Russia investigation, which looked into whether the Trump campaign colluded with Russian intelligence officials to interfere in the 2016 election, started before Trump took office.

The letter cites an op-ed Johnson wrote in The Hill last year in which he raised questions about potential “corrupt behavior” during the Obama administration.

“For these reasons, we respectfully request that you make available to us, as soon as possible, all information regarding the ‘unmasking’ of U.S. persons requested by members of President Obama’s administration around the time of the 2016 election through January 2017,” the lawmakers wrote.