In an unprecedented act of revenge against a prominent critic, the US president suddenly removed Mr Brennan's ability to access sensitive government information on Wednesday.
"I call it the rigged witch hunt, (it) is a sham," Mr Trump told the Wall Street Journal later that day. "And these people led it!"
"So I think it's something that had to be done."
However, the connection to special counsel Robert Mueller's investigation into Russian interference in the 2016 presidential election was not mentioned in an earlier statement by Mr Trump.
In the initial statement, the president denounced Mr Brennan's criticism of him and spoke of "the risks posed by his erratic conduct and behaviour." Mr Trump added he was fulfilling his "constitutional responsibility to protect the nation's classified information".
He also threatened to revoke the clearances of a host of former top intelligence and law enforcement officials, as well as a current member of the Justice Department. All are critics of the president or are people whom Mr Trump appears to believe are against him.
The statement was dated 26 July upon its release by the White House on Wednesday, which prompted some to suggest it had been decided weeks in advance and but was saved to use as a political distraction. Moments later, a second statement was released with the date removed.
Mr Trump's action against Mr Brennan marked an unprecedented politicisation of the federal government's security clearance process, critics and nonpartisan experts said. It also was a clear escalation in Mr Trump's battle with members of the US intelligence community, as Mr Mueller's investigation into possible collusion and obstruction of justice continues.
Democratic members of congress, reacting to Mr Trump's initial announcement, said his action smacked of an "enemies list" among fellow Americans and mimicked the behaviour of leaders in "dictatorships, not democracies."
Mr Brennan, in a phone interview with MSNBC, called the move an "abuse of power by Mr Trump".
"I do believe that Mr Trump decided to take this action, as he's done with others, to try to intimidate and suppress any criticism of him or his administration," he said, adding that he would not be deterred from speaking out.
Mr Trump, his statement read by his press secretary, accused Mr Brennan of having "leveraged his status as a former high-ranking official with access to highly sensitive information to make a series of unfounded and outrageous allegations, wild outbursts on the internet and television about this administration".
"Mr Brennan's lying and recent conduct characterised by increasingly frenzied commentary is wholly inconsistent with access to the nations' most closely held secrets," Mr Trump said.
In the WSJ interview, Mr Trump said he was prepared to yank Mr Brennan's clearance last week but that it was too "hectic". The president was on an extended working vacation at his New Jersey golf club last week.
Mr Brennan has indeed been deeply critical of Mr Trump's conduct, calling his performance at a press conference last month with Russian president Vladimir Putin in Finland "nothing short of treasonous".
Mr Brennan continued that criticism on Wednesday.
"I've seen this type of behaviour and actions on the part of foreign tyrants and despots and autocrats for many, many years during my CIA and national security career. I never, ever thought that I would see it here in the United States," he said.
Mr Brennan said he had not heard from the CIA or the Office of the Director of National Intelligence that his security clearance was being revoked, but learned it when the White House announced it. There is no requirement that a president has to notify top intelligence officials of his plan to revoke a security clearance. "The president has the ultimate authority to decide who holds a security clearance," the ODNI said in a statement.
Former CIA directors and other top national security officials are typically allowed to keep their clearances, at least for some period, so they can be in a position to advise their successors and to hold certain jobs.
Mr Trump's statement said the Brennan issue raised larger questions about the practice of allowing former officials to maintain their security clearances, and said that others officials' were under review.
They include former FBI director James Comey; James Clapper, the former director of national intelligence; former CIA director Michael Hayden; former national security adviser Susan Rice; and Andrew McCabe, who served as Mr Trump's deputy FBI director until he was fired in March.
Also on the list: fired FBI agent Peter Strzok, who was removed from the Russia investigation over anti-Trump text messages; former FBI lawyer Lisa Page, with whom Mr Strzok exchanged messages; and senior Justice Department official Bruce Ohr, whom Mr Trump recently accused on Twitter of "helping disgraced Christopher Steele 'find dirt on Trump'".
Mr Ohr was friends with Mr Steele, the former British intelligence officer commissioned by an American political research firm to explore Mr Trump's alleged ties with the Russian government. He is the only current government employee on the list.
At least two of the former officials, Mr Comey and Mr McCabe, do not currently have security clearances, and none of the eight receive intelligence briefings. Mr Trump's concern apparently is that their former status gives special weight to their statements, both to Americans and foreign foes.
Former intelligence officials are also wondering how far Mr Trump will go, according to a former senior intelligence official who spoke on condition of anonymity to share private conversations he's had with people who have worked in the field.
They said Mr Trump has moved from threatening to revoke security clearances of former intelligence officials who have not been involved in the Russia investigation to former officials who did work on the probe. And they wonder if he will next choose to target those who currently work on the investigation, which Mr Trump has called a "witch hunt."
The CIA referred questions to the White House.
Mr Clapper, reacting on CNN, called Mr Trump's actions "unprecedented," but said he didn't plan to stop speaking out. Asked what linked those threatened by the White House, Mr Clapper said he and the others have been outspoken about the Trump administration, have "directly run afoul of it" or have taken actions the president dislikes.
"So I guess that's what we all have in common," Mr Clapper said.
Sarah Huckabee Sanders, Mr Trump's press secretary, insisted the White House was not targeting only Trump critics. But Mr Trump did not order a review of the clearance held by former national security adviser Mike Flynn, who was fired from the White House for lying to vice president Mike Pence about his conversations with Russian officials and later pleaded guilty to lying to the FBI.
Democrats, and even some Republicans, lined up to denounce the president's move, with house minority leader Nancy Pelosi slamming it as a "stunning abuse of power." Senator Mark Warner, the top Democrat on the senate intelligence committee, warned a "dangerous precedent" was being set by "politicising the way we guard our national secrets just to punish the president's critics".
California representative Adam Schiff, the ranking Democrat on the house intelligence committee, tweeted, "An enemies list is ugly, undemocratic and un-American".
Maryland Democratic senatot Chris Van Hollen tweeted, "Trump is now categorising dissent and free speech as 'erratic behaviour."' He added, "Leaders behave like this in dictatorships, not democracies."
Several Republicans also weighed in, with senator Bob Corker saying: "Unless there's something tangible that I'm unaware of, it just, as I've said before, feels like a banana republic kind of thing."
House speaker Paul Ryan had previously dismissed Mr Trump's threat as nothing more than presidential "trolling".
Additional reporting by AP
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