The Republicans stormed into a hearing room where Laura Cooper, the U.S. defense official who oversees Ukraine and Russia matters, was due to testify behind closed doors and began yelling, lawmakers and aides said.

The impeachment inquiry focuses on Trump’s request for Ukraine to investigate a domestic rival - Democrat Joe Biden - for his personal political benefit.

In a dramatic confrontation during an escalating probe that threatens Trump’s presidency even as he seeks re-election next year, Capitol police were called in to clear the room and bring order, a Republican congressional aide said.

A witness inside the room said the Republicans brought cellphones into the high-security facility where electronic devices are forbidden.

“They’re freaked out. They’re trying to stop this investigation,” Democratic Representative Ted Lieu said. “They don’t want to hear from witness Cooper today. They know more facts are going to be delivered which are absolutely damning to the president of the United States.”