Alexander Vindman, a retired U.S. Army lieutenant colonel and former White House national security aide, is suing several allies of former President Donald Trump, alleging that they intimidated and retaliated against him while he was a key witness during Trump’s first impeachment.

According to the 73-page complaint, Vindman’s lawsuit “seeks long-overdue accountability for unlawful actions knowingly undertaken by close associates and allies” of Trump, alleging that they “engaged in an intentional, concerted campaign of unlawful intimidation and retaliation against [Vindman] to prevent him from and then punish him for testifying truthfully before Congress during impeachment proceedings against President Trump.”

Those named as defendants in Vindman’s suit include Donald Trump Jr., Trump’s eldest son; former Trump attorney Rudy Giuliani; former White House deputy chief of staff Dan Scavino; and former White House deputy communications director Julia Hahn. The complaint alleges that the defendants violated the Ku Klux Klan Act of 1871, which makes it unlawful to conspire to interfere with a federal official’s ability to carry out the duties of their office or to interfere with any witness’s ability to testify.

Vindman, who was formerly the National Security Council’s expert on Ukraine, had listened to a July 2019 phone call between Trump and Ukrainian President Volodymyr Zelensky, in which Trump asked Zelensky to investigate Joe Biden’s son Hunter Biden and his ties to Ukrainian businesses. Vindman reported the call through official channels, and Trump’s alleged attempts to pressure Ukraine into political investigations by leveraging promises of an official White House visit by Zelensky and military aid would later become the basis of his first impeachment and Senate trial.

Congress issued a subpoena to Vindman, who testified in an impeachment inquiry about his concerns over Trump’s actions involving Ukraine. Vindman immediately became the target of a witness-intimidation campaign by Trump and his allies that “did not simply happen by accident or coincidence,” his lawsuit alleges.

Representatives for Trump Jr. and Giuliani did not immediately respond to requests for comment.

Vindman alleges Trump and his allies coordinated strategies regarding impeachment witnesses, propagated false narratives about Vindman’s loyalty to the U.S., and leaked classified information to further their false narratives. They also publicly removed Vindman and his twin brother, a former NSC attorney, from their White House jobs.