WASHINGTON — The Trump administration failed on Tuesday to repatriate a Guantánamo Bay detainee to Saudi Arabia, missing a deadline set in his 2014 plea deal and dealing another blow to the struggling military commissions system at the wartime prison.

The detainee, Ahmed Muhammed Haza al-Darbi, agreed in February 2014 to plead guilty to terrorism-related offenses involving a 2002 attack by Al Qaeda on a French-flagged oil tanker off the coast of Yemen. Under the pretrial agreement, if he cooperated, he was to be sent home within four years to serve the remainder of his sentence in Saudi custody.