A dozen former US intelligence leaders have backed ex-CIA chief John Brennan after his security clearance was revoked by President Donald Trump.
The former CIA and national intelligence bosses called Mr Trump's move "an attempt to stifle free speech" in a joint statement on Thursday.
Mr Trump is keen to strip other high-ranking officials of access to classified information, US media say.
The nine people currently on a review list have all criticised the president.
Many have also been involved in the investigation into whether anyone on the Trump 2016 election campaign colluded with Russia.
Mr Trump has described the ongoing US Department of Justice inquiry as a "sham", repeatedly denying any collusion with Moscow.
He told the Wall Street Journal it was a "rigged witch hunt", led by "these people" from the US intelligence community.
On Friday, Mr Trump told reporters he "never respected" Mr Brennan and that he has "had a tremendous response" for revoking his security clearance.
Mr Brennan, an outspoken critic of Mr Trump, has been locked in a war of words with the president this week after arguing the Russia investigation was "well-founded".
The White House said removing his access to sensitive government information on Wednesday was due to Mr Brennan's "erratic conduct" and "outrageous allegations".
Mr Brennan then hit back in a New York Times op-ed, saying Mr Trump's claims of no collusion were "hogwash".
He said Mr Trump was trying to "scare into silence" other critics.
However, Richard Burr, head of the Senate Intelligence Committee, which is generally agreed to have led a non-partisan investigation of the Russia allegations, backed Mr Trump's revoking Mr Brennan's clearance.
The Republican said: "Director Brennan's recent statements purport to know as fact that the Trump campaign colluded with a foreign power."
He added: "If his statement is based on intelligence he has seen since leaving office, it constitutes an intelligence breach.
"If he has some other personal knowledge of, or evidence of, collusion, it should be disclosed to the special counsel, not The New York Times."
Former top intelligence officials typically retain their clearances for a time, often advising successors.
William McRaven, a retired Navy admiral who oversaw the raid that resulted in the death of Osama bin Laden, also wrote an open letter to Mr Trump on Thursday, saying he would consider it "an honour" if his own clearance were removed.
"Through your actions, you have embarrassed us in the eyes of our children, humiliated us on the world's stage and, worst of all, divided us as a nation," Mr McRaven wrote in the Washington Post.
"If you think for a moment that your McCarthy-era tactics will suppress the voices of criticism, you are sadly mistaken. The criticism will continue until you become the leader we prayed you would be."
The 13 signatories of the joint statement - who include former CIA directors Robert Gates, David Petraeus, Leon Panetta and Porter Goss - noted that while they may not all agree with Mr Brennan's comments, they supported his right to voice an opinion.
The ex-officials called Mr Trump's move against Mr Brennan "ill-considered and unprecedented".
Two of them - ex-director of National Intelligence James Clapper and former CIA director Michael Hayden - are also on the White House list of those whose security clearances have been publicly threatened.
Mr Clapper and Mr Brennan were among the Obama administration officials who briefed Mr Trump before his inauguration on evidence of Russian meddling in the 2016 election.
Mr Trump has repeatedly urged Attorney General Jeff Sessions and other officials to end the Russia investigation led by Special Counsel Robert Mueller.