The congressional panel investigating last year's Capitol riot has issued a legal summons ordering former President Donald Trump to testify to lawmakers.
Addressing him, the document reads: "You were at the center of the first and only effort by any US President to overturn an election."
It goes on: "You knew this activity was illegal and unconstitutional."
A lawyer for Mr Trump accused the lawmakers on the committee of "flouting norms".
The former president has lambasted the inquiry as a ruse designed to distract voters from the "disaster" of Democratic governance with US midterm elections looming next month.
Mr Trump could face criminal charges if he does not comply with the subpoena.