Supreme Court Justice Clarence Thomas, 75, has raised alarms after being absent from court on Monday with no explanation.

The eldest and longest-serving Supreme Court Justice’s absence was announced by Chief Justice John Roberts, who said his colleague would still be able to participate in the day’s cases using briefs and transcripts of the arguments presented before the high court.

Thomas did not participate remotely in arguments, however, as justices sometimes do when they are ill or otherwise unable to attend court in person, AP reported.

The court heard arguments in two cases on Monday: Snyder v. United States — which concerns a federal anti-bribery statute — and Chiaverini v. City of Napoleon, Ohio — a Fourth Amendment case.

Thomas’ mysterious absence comes two years after the President George H.W. Bush appointee was hospitalized with an infection — during which he still took part in cases.

The conservative jurist has served since Oct. 23, 1991 and is considered a stalwart member of the court’s 6-3 conservative majority.

During his tenure, Thomas has come under intense scrutiny amid allegations he’d failed to properly disclose luxury trips with influential Republican megadonor Harlan Crow and numerous other potential ethics breaches.

Last month, Thomas’ role in putting Republican presidential candidate Donald Trump back on primary ballots in multiple states was also met with criticism.

Trump had been removed in accordance with Section 3 of the Constitution — which bars former officeholders who “engaged in insurrection” from ever holding office again.