Navarro was defiant in remarks to reporters before he headed to a federal lockup 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 defying a subpoena for documents and a deposition from the U.S. House Jan. 6 committee that investigated the 2021 capitol attack. He served as a White House trade advisor under then-President Trump and later promoted the Republican’s baseless claims of mass voter fraud in the 2020 election that the incumbent president lost to Democrat Joe Biden.

Navarro has maintained that he couldn’t cooperate with the committee because Trump had invoked executive privilege. Courts have rejected that argument, finding Navarro couldn’t prove Trump had actually invoked it.

“When I walk in that prison today, the justice system — such as it is — will have done a crippling blow to the constitutional separation of powers and executive privilege,” Navarro told reporters Tuesday across the street from the prison.

Navarro then got in a car with his lawyer to head to the lockup, and the federal Bureau of Prisons confirmed later Tuesday that Navarro was in custody.

Navarro had asked to stay free while he appealed his conviction to give the courts time to consider his challenge. But Washington’s federal appeals court denied his bid to stave off his sentence, finding his appeal wasn’t likely to reverse his conviction.

And Supreme Court Chief Justice John G. Roberts Jr. on Monday also refused to step in, saying in a written order that he has “no basis to disagree” with the appeals court. Roberts said his finding doesn’t affect the eventual outcome of Navarro’s appeal.

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

The House committee spent 18 months investigating the insurrection, interviewing over 1,000 witnesses, holding 10 hearings and obtaining more than 1 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.

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