White House acting chief of staff Mick Mulvaney instructed a former security clearance official not to testify before the House Oversight Committee, according to letters sent to panel chairman Elijah Cummings Monday, and first obtained by ABC News.

The big picture: Former White House Personnel Security Director Carl Kline was to testify before the committee Tuesday, after the panel voted to subpoena him in its White House security clearance investigation.

Details: In the White House letter, Michael Purpura, deputy counsel to President Trump, wrote Kline was instructed not to appear after the committee rejected a White House request for an Office of Counsel of the President representative to attend the deposition.

What they're saying: Kline's lawyer wrote a letter stating they wished to answer the "legitimate legislative questions of this committee, but [were] warned of impending conflict."