There are growing Democrat calls for impeachment proceedings against US President Donald Trump, after a former White House counsel failed to appear before Congress despite a subpoena.
Donald McGahn skipped a hearing on Tuesday about the Mueller report.
Mr Trump has vowed to block all subpoenas of current and former staff, sparking outrage from the Democrats.
House Speaker Nancy Pelosi has scheduled a party meeting on Wednesday to discuss impeachment.
The Trump administration says the report by Special Counsel Robert Mueller into allegations of Russian collusion and obstruction of justice cleared the president of wrongdoing, and that there are no further questions to answer.
In a tweet, President Trump argued he had "allowed everyone to testify" to Robert Mueller's team and accused the Democrats of seeking a "do-over" of the investigation.
On the subpoenas - an order that a person attend court - the White House and the Department of Justice both say staff cannot be forced to testify.
But House Judiciary Committee chairman Jerry Nadler said their subpoenas "are not optional" and said they would hear Mr McGahn's testimony "even if we have to go to court to secure it".
Democratic Party leaders have so far held off pressure from their lawmakers to begin impeachment proceedings, and the party is divided over the merits of the move, but the pressure is growing as the president stonewalls congressional inquiries.
Democratic representative Mark Pocan - one of the leaders of the party's progressive wing - said the president's latest moves had enhanced his "appearance of guilt" and "pushed Congress to a point where we must start an impeachment inquiry".
"We need to do our job and vote on impeachment," tweeted Representative Alexandria Ocasio-Cortez.
And Republican congressman Justin Amash broke ranks, tweeting that the president's behaviour met "the threshold for impeachment" but he has been criticised by fellow party members.
Asked about impeachment on Tuesday, Democratic House Majority Leader Steny Hoyer, said: "I don't think we're there at this point in time."
A letter to the House of Representatives Judiciary Committee said Mr McGahn was "absolutely immune from compelled Congressional testimony" and said Mr Trump had directed him not to appear on Tuesday.
Mr McGahn - who resigned after almost two years as White House counsel in October 2018 - said he would respect the president's instruction.
In the wake of Mr McGahn's failure to appear, Mr Nadler announced he had issued subpoenas to former White House Communications Director Hope Hicks and Annie Donaldson, Mr McGahn's former chief of staff.
According to a New York Times report, Democratic lawmakers and aides were examining possible rules changes to allow the House to fine people held in contempt, as well as other means to break the impasse.
Presidential administrations have challenged subpoenas before, but analysts say President Trump's vow to block all subpoenas issued by the House Judiciary Committee in relation to the Mueller report is unprecedented.