The US ambassador to the European Union is currently testifying in the ongoing impeachment inquiry against President Donald Trump.
It is expected Gordon Sondland will be questioned about text messages from the US ambassador to Ukraine, Bill Taylor, discussing Trump's effort to pressure Ukraine to investigate his rival, Joe Biden.
On Wednesday, a former senior adviser to US Secretary of State Mike Pompeo was the latest person to testify. Michael McKinley was quizzed for five hours by House committees after abruptly resigning last week.
Politicians are examining whether Trump improperly pressured Ukraine to launch an investigation into Biden, a leading candidate for the 2020 Democratic presidential nomination.
The US House Intelligence Committee chairman says testimony transcripts will be made public.
Joel Rubin is president of the Washington Strategy Group - an advisory firm specialising in national security and foreign policy.