Kurt Volker, the former US special representative to Ukraine, was on a list of witnesses requested to appear by Republican members of the Intelligence Committee, according to NPR.
In earlier statements he made in closed-door testimony, Volker contradicted Trump's claims that Biden urged Ukraine to fire a prosecutor who was investigating a natural-gas company that Hunter Biden was working for.
Volker said in his testimony that the prosecutor, Viktor Shokin, was not investigating the natural-gas company or any other corruption cases.
Volker said that Joe Biden was execution US policy in pushing for Shokin to be fired.
Additionally, Volker said in testimony that Trump's personal lawyer, Rudy Giuliani, was a "direct conduit" between Trump and Ukraine.