Thomas, 75, was in his usual seat, to the right of Chief Justice John Roberts as the court met to hear arguments in a case about the Capitol riot on Jan. 6, 2021.

Thomas has ignored calls from some progressive groups to step aside from cases involving Jan. 6 because his wife, Ginni, attended then-President Trump’s rally near the White House before protesters descended on the Capitol. Ginni Thomas, a conservative activist, also texted senior Trump administration officials in the weeks after the election offering support and reiterating her belief that there was widespread fraud in the election.

On Monday, Roberts announced Thomas’ absence without providing an explanation. Justices sometimes miss court, but participate remotely. Thomas did not take part in Monday’s arguments.

He was hospitalized two years ago with an infection, causing him to miss several court sessions. He took part in the cases then, too.

Thomas is the longest serving of the current justices, joining the Supreme Court in 1991.