Retired Col. Yevgeny “Eugene” Vindman, who blew the whistle on the 2019 call between former President Trump and Ukrainian President Volodymyr Zelensky that resulted in Trump’s first impeachment, said on Saturday that Russian leaders will “never escape accountability” for their crimes in Ukraine.

“They can never escape accountability, no matter how long it takes,” Vindman told MSNBC.

Vindman, who was born in Ukraine, is currently investigating war crimes in the country on behalf of the Atrocity Crimes Advisory Group, a joint initiative created by the United States, United Kingdom and European Union.

“At this stage, investigation is the most important thing,” Vindman said. “Accountability is a long-term process. [For] World War II, Germany was still holding war crimes trials as recently as last year. Same thing is happening in Yugoslavia for the crimes that were committed decades ago.

“So, at this stage, it’s about collecting the evidence and preparing cases for when the opportunity presents itself,” he added.

Vice President Kamala Harris announced last weekend while speaking at the Munich Security Conference that the U.S. had formally determined that Russia has committed crimes against humanity in Ukraine.

“The United States has formally determined that Russia has committed crimes against humanity, and I say to all those who have perpetrated these crimes, and to their superiors, who are complicit in these crimes, you will be held to account,” Harris said.