Washington (CNN) Ninety former US national security officials wrote a letter Sunday praising the anonymous whistleblower whose recent complaint about President Donald Trump's communications with Ukraine touched off a firestorm, leading House Democrats to launch an impeachment inquiry into Trump.

"While the identity of the whistleblower is not publicly known, we do know that he or she is an employee of the U.S. Government. As such, he or she has by law the right—and indeed the responsibility—to make known, through appropriate channels, indications of serious wrongdoing," the former officials from both Democratic and Republican administrations wrote. "That is precisely what this whistleblower did; and we applaud the whistleblower not only for living up to that responsibility but also for using precisely the channels made available by federal law for raising such concerns."

Among the scores of signatories on the letter is former Defense Secretary Chuck Hagel, former CIA Directors John Brennan and Michael Hayden, who is a CNN analyst, and former Director of National Intelligence James Clapper, who is a CNN contributor.

The letter comes amid an intensifying focus over the whistleblower's complaint about Trump's July call with Ukrainian President Volodymyr Zelensky and the actions the White House took to conceal the records of the conversation.

In an attempt to discredit the whistleblower, Trump and other Republicans have launched partisan attacks on the individual, who, according to a source familiar with the ongoing investigation, is a registered Democrat. Trump demanded to meet his "accuser" face to face. The whistleblower's lawyer wrote a letter to the chairmen and ranking members of the House and Senate Intelligence Committees and acting Director of National Intelligence Joseph Maguire to express "serious concerns" for his client.