Three House Democrats who chair impeachment-related committees warned Secretary of State Mike Pompeo to “immediately cease intimidating Department witnesses in order to protect himself and the President.”

The statement from House Intelligence Committee Chairman Adam Schiff of California, Foreign Affairs Committee Chairman Eliot Engel of New York, and Oversight Committee Chairman Elijah Cummings of Maryland, came Tuesday after Pompeo told Democrats he would not allow State Department officials to testify in their impeachment inquiry.

“Any effort to intimidate witnesses or prevent them from talking with Congress — including State Department employees — is illegal and will constitute evidence of obstruction of the impeachment inquiry,” the lawmakers wrote. “In response, Congress may infer from this obstruction that any withheld documents and testimony would reveal information that corroborates the whistleblower complaint.”

The congressmen cited the impeachment inquiry as part of their authority to order testimony.

The House never voted to open an impeachment inquiry, but Speaker Nancy Pelosi of California declared the House is conducting one.

Democrats are probing President Trump’s July 25 call with Ukrainian President Volodymyr Zelensky and the president’s request that Zelensky investigate former Vice President Joe Biden.

Pompeo was also on the call and lawmakers want to interview him. Pompeo accused Democrats of trying to “bully” State Department officials.