The U.S. State Department will do everything necessary to determine whether former U.S. Ambassador Marie Yovanovitch was under threat in Ukraine, Secretary of State Mike Pompeo said Friday.

Documents released this week indicated Lev Parnas, a Ukraine-born U.S. citizen, helped U.S. President Donald Trump's personal lawyer Rudy Giuliani investigate U.S. presidential candidate Joe Biden and his son Hunter.

They also showed Parnas was involved in monitoring Yovanovitch's movements before Trump removed her in May. Yovanovitch, a respected career diplomat, was a key witness in the U.S. House of Representatives' investigation of Trump before he was impeached in December.