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(CNN) —  

Over a dozen former senior intelligence officials – a group that includes officials who served under both Democratic and Republican presidents – are denouncing President Donald Trump’s decision to revoke former CIA Director John Brennan’s security clearance as “ill-considered” and “unprecedented.”

With the exception of Brennan – and excluding those who served under Trump – every CIA director since George Tenet, who took over the agency in 1996, signed the letter.

“We all agree that the president’s action regarding John Brennan and the threats of similar action against other former officials has nothing to do with who should and should not hold security clearances – and everything to do with an attempt to stifle free speech,” the former senior intelligence officials, including former CIA directors, former CIA deputy directors, and a former director of national intelligence, said in a statement released Thursday.

They went on to argue, “We have never before seen the approval or removal of security clearances used as a political tool, as was done in this case.”

“Beyond that, this action is quite clearly a signal to other former and current officials,” the former officials said, adding that the signal “is inappropriate and deeply regrettable.”

The list originally included 12 officials, but overnight, former CIA director and Defense Secretary Robert Gates added himself to the list. Former Director of National Intelligence Dennis Blair and former Deputy CIA Director Bert Calland also signed onto the statement Friday.

One of the signees, former director of the CIA and NSA Michael Hayden told CNN’s John Berman Friday on “New Day” that making the decision to sign the statement was not difficult.

“Using the security clearance process to punish a political opponent was simply inappropriate even though we admit that the president has absolute authority in this area,” Hayden said. “It’s just a bad thing to do for the health of the American republic and the health of American debate.”

The former intelligence leaders added in their statement that “decisions on security clearances should be based on national security concerns and not political views.”

They also defended Brennan’s character and called allegations against Brennan of wrongdoing “baseless.”

Hayden and James Clapper, another official on the statement and the former Director of National Intelligence under President Barack Obama, are both included on a list of officials that the administration is considering stripping of security clearances. Both have been outspoken against Trump.

Trump announced on Wednesday that he was revoking Brennan’s security clearance, first citing that the former CIA director posed a security risk. He later, in an interview with the Wall Street Journal, connected Brennan’s security clearance revocation with the Russia investigation, saying the probe was a “witch hunt” and “these people led it.”

Trump told reporters Friday that he’s received a “tremendous response” after revoking Brennan’s clearance.

Following the President’s comments, former CIA director and onetime Trump campaign adviser James Woolsey issued a separate statement in which he said “objective criteria” should be used in evaluating who gets to keep a security clearance.

Woolsey was not able to review the joint statement issued by the other former intelligence chiefs by the deadline, and chose to write his own statement, according to a person with knowledge of the decision.

The President said he would “soon” revoke the clearance of Bruce Ohr, a Justice Department official whom Trump has criticized for his connections to a dossier of infamous allegations about the US President and Russia. The dossier – portions of which remain unconfirmed – was commissioned as opposition research by political opponents of then-candidate Trump and compiled by a former British intelligence agent. Ohr was demoted upon the discovery of certain meetings with Fusion GPS founder Glenn Simpson, whose firm was behind dossier, and Christopher Steele, the former British intelligence officer who assembled it.

The mention of Ohr on Friday is notable because he currently works at the Justice Department and has not publicly criticized Trump.

Brennan, who served under Obama, was one of the intelligence chiefs who signed off on the intelligence community’s January 2017 assessment that Russia interfered with the intent to help Trump and to hurt Hillary Clinton.

This story has been updated.

CNN’s Jim Sciutto, Abby Phillip, Elizabeth Landers, Barbara Starr and Jamie Gangel contributed to this report.