KYIV, Ukraine (AP) — The top U.S. diplomat in Ukraine, William Taylor, is scheduled to appear Tuesday before impeachment investigators in Congress.

Taylor, a former Army officer and retired career civil servant, has emerged as an unlikely hero in the impeachment inquiry of President Donald Trump. Taylor was drawn into the Trump administration effort to leverage U.S. military aid for Ukraine, an effort Taylor believed was improper and tried to stop.

In July, Trump had his now-famous phone conversation with Ukrainian President Volodymyr Zelenskiy in which he pressed him to investigate unsubstantiated claims about Democratic rival Joe Biden and look into the 2016 presidential election.

In a follow-up to the call, Taylor exchanged texts with two of Trump's point men on Ukraine and expressed concern about conditioning U.S. aid to Ukraine on the investigations.