White House officials who worked with former national security adviser John Bolton were frustrated and surprised when White House acting chief of staff Mick Mulvaney signed onto a lawsuit that opposes him and others close to President Donald Trump cooperating with the impeachment inquiry.

The Washington Post reported Sunday, two days after Mulvaney joined the lawsuit through his personal lawyer, that he played a key role in Ukraine policy — which is at the center of the impeachment probe that seeks to uncover whether Trump used his office for political gain by using U.S. military aid to Ukraine as a weapon.

The Post said people close to Bolton and his former deputy Charles Kupperman were taken back when news broke that Mulvaney was joining the suit. Bolton, according to the report, sees Mulvaney as having a central role in the military aid situation. The two were hardly speaking to each other when Bolton departed the Trump administration in September, the Post noted.

Neither Bolton nor Mulvaney complied with House subpoenas to testify last week as part of the Democrats-run impeachment probe. Bolton has indicated he is willing to talk if the court system compels him to do so. But Mulvaney's decision to join the lawsuit indicates he intends to fight the subpoena on the grounds that a directive from the White House not to testify takes precedence over a subpoena from Congress.

"As acting chief of staff, Mr. Mulvaney intends to follow any lawful order of the president and has no reason to think that the order at issue is unlawful — other than the fact the House has threatened him with charges of contempt and obstruction for following it," Mulvaney's lawyer William Pittard told the Post.

Trump reportedly ordered aid to Ukraine held up after asking its president, Volodymyr Zelenskiy, in July to investigate former U.S. Vice President Joe Biden and his son Hunter for corruption. Biden is one of Trump's main Democrat rivals in the 2020 election.

Bolton signed a book deal in recent weeks worth a reported $2 million.