The former U.S. Army officer who became a star witness during former President Donald Trump’s first impeachment over a pressure campaign on Ukraine sued four allies of the former president on Wednesday, alleging they conducted a campaign of witness intimidation and retaliation.

Retired Lt. Col. Alexander Vindman filed a civil rights lawsuit in D.C. federal court, alleging that Trump’s allies mounted an “intentional, concerted campaign of unlawful intimidation and retaliation” against him in order to “prevent him from and then punish him for testifying truthfully before Congress” during the impeachment investigation.