U.S. Ambassador to the E.U. Gordon Sondland is easily impeachment investigators’ most-wanted witness. After amending his initial testimony to say he simply presumed Trump sought investigations from Ukraine in exchange for security aid, a US embassy official in Kyiv testified that he heard Sondland on the phone with the president, who directly pressed him on the status of the plan. Lawmakers on both sides will likely have tough questions for the hotelier turned Trump-backing diplomat with a messy role at the center of this story.

Lt. Col. Alexander Vindman, a Soviet-born war hero, is one of the three people investigators will hear from who directly listened to Trump’s July 25 call with Zelensky. The former National Security Council official is expected to offer more details about that, and also to set the scene of a July 10 White House meeting in which Sondland connected “investigations” to favor with Trump for a visiting Ukrainian delegation; afterward, Vindman immediately notified the NSC’s top attorney.

Tim Morrison succeeded Fiona Hill as the top official for Ukraine issues at the National Security Council. He also listened to the July 25 call, and while Vindman found it troubling, Morrison told investigators in his closed-door deposition that he did not believe anything Trump raised was illegal. Republicans consider him one of their strongest witnesses and specifically requested his testimony; however, Democrats still believe he will confirm the key elements of their quid-pro-quo case.