Two White House officials skipped their scheduled impeachment inquiry testimony behind closed doors Tuesday, reportedly defying their subpoenas to appear.

In a new U.S. Department of Justice (DOJ) memo that came to light on Tuesday, the Trump administration argued that Democrat impeachment investigators’ attempt to compel testimony from executive branch officials are “legally invalid” unless they allow government attorneys to accompany the witnesses.

“The assistance of agency counsel” is needed because testimony has the potential to disclose information “protected by executive privilege,” the DOJ Office of Legal Counsel noted in a memo dated November 1.

Wells Griffith, a special assistant to Trump and energy adviser on the National Security Council (NSC), and Michael Duffey, associate director for national security programs at the Office of Management and Budget (OMB), did not appear for their Tuesday depositions.

Their refusal to testify came a day after four White House officials defied subpoenas for testimony issued by House Democrat impeachment investigators.

The White House has told House Democrat leaders it will not cooperate in the impeachment probe.

Roll Call suggested that both Griffith and Duffey defied a subpoena to appear.

According to the Washington Post, House Democrats are seeking testimony from Duffey because he “was allegedly involved in approving orders to hold back nearly $400 million in congressionally approved military aid for Ukraine.”

Democrats also want a deposition from Griffith because he allegedly “took part at a key meeting at the White House with Ukrainian officials in July,” the Post added.

“We may infer by the White House obstruction here, that their testimony would be further incriminating of the President,” House Intelligence Committee Chairman Adam Schiff (D-Calif.) said Monday, referring to the four White House officials who defied their subpoenas that day.

In its five-page memo, the DOJ argues that House Democrat’s subpoenas that ask witnesses to appear without counsel are “legally invalid,” noting:

The [House Intelligence Committee] could address this separation of powers problem by allowing agency counsel to assist the employee during the deposition. Should the committee not do so, however, a subpoena purporting to require a witness to appear without such assistance would be invalid and not subject to civil or criminal enforcement.

The Democrat-led House Committees on Oversight and Reform, Intelligence, and Foreign Affairs are conducting the impeachment probe.