This article was produced in partnership with the Pulitzer Center on Crisis Reporting.

WASHINGTON — The military judge presiding in the Sept. 11 death penalty trial at Guantánamo Bay, Cuba, has scheduled his retirement for later this year, in the latest blow to efforts to start the long-running trial in 2021.

The judge, Col. W. Shane Cohen, wrote in a one-page letter to the chief war court judge that he was ending his 21 years of Air Force service on July 1. Unless another judge is appointed sooner, he wrote, April 24 would be his last day presiding in the case against Khalid Shaikh Mohammed and four other men who are accused of orchestrating the Sept. 11, 2001, attacks that killed 2,976 people in New York, Pennsylvania and at the Pentagon.

The looming departure of the judge, coupled with a current shutdown of legal team access to the United States Navy base prison zone because of the coronavirus pandemic, cast a shadow on the prospects of meeting the target start date of Jan. 11, 2021. A new judge has to be chosen, and he or she will need time to read the more than 33,150-page transcript of the case as well as hundreds of legal filings, some still awaiting rulings.

Colonel Cohen’s decision to leave the case also comes as he has been hearing testimony — and has scheduled more witnesses — in an ongoing set of hearings on the defense lawyers’ requests to exclude from the trial the F.B.I. interrogations of the men in 2007. The defense lawyers say those interrogations are tainted by the torture the defendants endured during their three and four years in secret C.I.A. prisons.