White House Chief of Staff Reince Priebus, right, watches as White House Staff Secretary Rob Porter, center, hands President Donald Trump a confirmation order for James Mattis as defense secretary, Jan. 20, 2017, in the Oval Office of the White House in Washington. (File photo: AP)

The US House Judiciary Committee has subpoenaed former White House staff secretary Rob Porter as part of its investigation into President Donald Trump’s conduct in office and whether he should be impeached.

The subpoena demands that Porter testify at a Sept. 17 hearing. The committee previously subpoenaed former Trump campaign manager Corey Lewandowski and former White House aide Rick Dearborn to appear that same day. All three men are featured prominently in former special counsel Robert Mueller’s report on Russia and Trump’s possible obstruction of justice.

Mueller said he could not exonerate Trump on obstruction and has indicated he believes the matter is now up to Congress.

The Judiciary panel has sued the Trump administration for blocking previous witnesses. The White House says those witnesses are “absolutely immune” from testimony.