Supreme Court Justice Clarence Thomas was absent from the court Monday with no explanation.

Thomas, 75, also was not participating remotely in arguments, as justices sometimes do when they are ill or otherwise can’t be there in person.

Chief Justice John Roberts announced Thomas’ absence, saying that his colleague would still participate in the day’s cases, based on the briefs and transcripts of the arguments. The court sometimes, but not always, says when a justice is out sick.

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

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