Former Trump White House official Peter Navarro reported to prison Tuesday to begin serving his sentence for refusing to cooperate with a congressional investigation into the Jan. 6, 2021, attack on the U.S. Capitol.

Navarro was defiant in his comments to reporters before heading to a federal prison in Miami, where he will serve a four-month sentence after being found guilty of contempt of Congress charges.

Navarro was found guilty in September of ignoring a subpoena for documents and a Jan. 6 statement from the U.S. House of Representatives committee investigating the 2021 attack on the Capitol. Under then-president, he served as trade adviser to the White House

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Trump and later promoted Republicans’ baseless claims of massive voter fraud in the 2020 election, which the incumbent president lost to Democrat Joe Biden.

Navarro has maintained that he could not work with the committee because Trump had invoked executive privilege. Courts have rejected that argument, finding that Navarro could not prove that Trump had actually invoked it.

When I walk into that prison today, the justice system as it stands now will have dealt a crippling blow to the constitutional separation of powers and executive privilege, Navarro told reporters across the street from the prison on Tuesday.

Navarro then got into a car with his attorney to go to jail, and the federal Bureau of Prisons confirmed later Tuesday that Navarro was in custody.

Navarro had asked to remain free while he appealed his conviction to give the courts time to consider his challenge. But the federal appeals court in Washington rejected his attempt to divert his sentence, saying his appeal was unlikely to overturn his conviction.

And Supreme Court Chief Justice John G. Roberts Jr. also declined to intervene Monday, saying in a written order that he has no basis for disagreeing with the appeals court. Roberts said his findings will not affect the ultimate outcome of Navarro’s appeal.

Navarro was the second Trump aide to be convicted of contempt of Congress charges. Former White House adviser Stephen K. Bannon previously received a four-month prison sentence, but another judge allowed him to remain free pending the appeal.

The House committee spent eighteen months investigating the insurrection, interviewing more than a thousand witnesses, holding ten hearings and obtaining more than one million pages of documents. In its final report, the panel ultimately concluded that Trump criminally engaged in a multi-part conspiracy to overturn the election results and failed to act to stop his supporters from storming the Capitol.

Adriana Gomez Licon writes for the Associated Press. AP reporter Alanna Durkin Richer contributed from Boston.