Mueller's final report in April outlined 10 acts of alleged obstruction by the president
A Washington appeals court judge ruled Monday that President Donald Trump's top aides must comply with subpoenas issued by the impeachment investigation against him.
In a case involving former White House counsel Don McGahn, who was subpoenaed in May by the House Judiciary Committee, Judge Ketanji Jackson ruled that senior administration officials cannot claim absolute immunity from testifying based on their closeness to the US leader.
While Jackson ruled specifically on McGahn's case, she said it had broad application to all current and former presidential aides.
"Presidents are not kings," Jackson wrote in her decision.
"No one, not even the head of the Executive branch, is above the law."
Watch: Will US President Donald Trump be impeached?
She said Congress has the power to subpoena any of the president's advisors, whether they are involved in domestic policies or sensitive national security issues.
"It goes without saying that the law applies to former White House Counsel Don McGahn, just as it does to other current and former senior-level White House officials," she wrote.
That ruling could open a path for the House Intelligence Committee, which is building a case for impeachment charges against Trump based on his pressuring Ukraine for domestic political help, to force testimony from three top witnesses: former national security advisor John Bolton, White House Chief of Staff Mick Mulvaney and Secretary of State Mike Pompeo.
All are believed to have direct knowledge of Trump's alleged demand that Ukraine President Volodymyr Zelensky open investigations into his Democratic rivals in exchange for a high profile summit and much-needed military aid.
The ruling could also give fresh life to the stalled impeachment case in the Judiciary Committee, which has tried to develop charges against Trump for obstructing the Russian collusion investigation led by Special Counsel Robert Mueller.
Mueller's final report in April outlined 10 acts of alleged obstruction by the president.
"Don McGahn is a central witness to allegations that President Trump obstructed Special Counsel Mueller's investigation," Judiciary Committee Chairman Jerry Nadler said in a statement.
"The administration's claim that officials can claim 'absolute immunity' from Congressional subpoenas has no basis in law, as the court recognized today."