The House Judiciary Committee subpoenaed ex-White House staff secretary Rob Porter as part of its investigation into President Trump following the release of special counsel Robert Mueller’s report.

House Judiciary Committee Chair Jerry Nadler (D-NY) announced the subpoena in a press release on Monday. Porter has been asked to appear for testimony on Sept. 17, the same day the committee compelled Corey Lewandowski and Rick Dearborn to appear.

“As I’ve said before, any other American would have been prosecuted based on the evidence special counsel Mueller uncovered in his report,” Nadler said in the statement. “Rob Porter was prominently featured in the special counsel’s description of President Trump’s efforts to obstruct justice by directing then-White HouseCounsel Don McGahn to fire the Special Counsel, and then ordering him to lie about it.”

Porter was a key witness in the obstruction of justice portion of Mueller’s probe and his testimony would be part of the committee’s efforts to mull whether to recommend article of impeachment against Trump. Porter resigned from the White House after he was accused of physically abusing two of his ex-wives, but was around during several potential obstruction incidents that Mueller documented in his report.

The Mueller report claims that Trump ordered Porter to ask White House counsel Don McGahn to create a document that would prove Trump didn’t ask McGahn to fire Mueller. Porter also told McGahn that Trump would fire him if he didn’t combat press reports about Trump asking him to fire the special counsel.

The White House will likely block Porter’s testimony as it has with every witness the committee subpoenas to appear, arguing former aides have sweeping immunity from testifying before Congress. House Democrats are challenging that argument in court.