Former CIA director John Brennan struck back at President Trump on Thursday, writing in a blistering op-ed that Trump’s claims of no collusion with Russia in the 2016 presidential election are “hogwash.”

The piece, posted on the New York Times website, came a day after Trump announced he was stripping the security clearance of Brennan, who led the CIA during most of President Barack Obama’s second term and has emerged as one of Trump’s fiercest critics.

In his op-ed, Brennan characterized Trump’s action as “politically motivated” and said it was “an attempt to scare into silence others who might dare to challenge him.”

Brennan wrote that it is “critically important” that special counsel Robert S. Mueller III be permitted to complete his investigation into Russian interference in the 2016 election and whether Trump’s campaign coordinated with Russia.

While Trump has repeatedly insisted there was “no collusion,” Brennan wrote that the “only questions that remain are whether the collusion that took place constituted criminally liable conspiracy, whether obstruction of justice occurred to cover up any collusion or conspiracy, and how many members of ‘Trump Incorporated’ attempted to defraud the government by laundering and concealing the movement of money into their pockets.”

The former CIA director took aim at Trump for his public call as a candidate in July 2016 for Russia to help find missing emails of his Democratic opponent, Hillary Clinton, who was being scrutinized for having used a private email server while secretary of state.

“By issuing such a statement, Mr. Trump was not only encouraging a foreign nation to collect intelligence against a United States citizen, but also openly authorizing his followers to work with our primary global adversary against his political opponent,” Brennan wrote.

“Such a public clarion call certainly makes one wonder what Mr. Trump privately encouraged his advisers to do — and what they actually did — to win the election,” Brennan said. “While I had deep insight into Russian activities during the 2016 election, I now am aware — thanks to the reporting of an open and free press — of many more of the highly suspicious dalliances of some American citizens with people affiliated with the Russian intelligence services. Mr. Trump’s claims of no collusion are, in a word, hogwash.”

Trump said Wednesday that he is also reviewing the security clearances of nine other former officials, including former FBI director James B. Comey, all of whom have criticized the president or been targeted by congressional Republicans seeking to discredit the Russia probe.

Trump appeared to make a direct link in an interview with the Wall Street Journal on Wednesday. “I call it the rigged witch hunt,” Trump said of the Russia probe. “And these people led it. So I think it’s something that had to be done.”

David Nakamura and Felicia Sonmez contributed to this report.