(CNN) Democratic House impeachment investigators have invited former national security adviser John Bolton to testify behind closed doors next week, according to two sources familiar with the matter, but his attorney said he won't appear without a subpoena.

The committees have also invited National Security Council lawyer John Eisenberg and his deputy Michael Ellis to appear on Monday, according to the sources.

If he appears before Congress, Bolton would be the most senior witness to testify in the impeachment inquiry to date, and he's at the center of several big events related to the freezing of Ukraine aid and the push for Ukraine to investigate its possible role in the 2016 US election along with Vice President Joe Biden and his son Hunter.

Asked whether he would issue a subpoena to Bolton, House Intelligence Chairman Adam Schiff would not say. "We certainly hope that he will cooperate. He obviously has very relevant evidence to provide," the California Democrat said.