In this Nov. 21, 2013, photo reviewed by the U.S. military, dawn arrives at the now closed Camp ­X-Ray, which was used as the first detention facility for suspected militants at Guantanamo Bay. (Charles Dharapak/AP)

Military prosecutors this year learned about a massive cache of CIA photographs of its former overseas “black sites” while reviewing material collected for the Senate investigation of the agency’s interrogation program, U.S. officials said.

The existence of the approximately 14,000 photographs will probably cause yet another delay in the military commissions at Guantanamo Bay, Cuba, as attorneys for the defendants demand that all the images be turned over and the government wades through the material to decide what it thinks is relevant to the proceedings.

Defense attorneys said they have not yet been informed about the photographs and said it is unacceptable that they should come to light now, more than three years after the arraignment of Khalid Sheik Mohammed and four other defendants accused of planning the Sept. 11, 2001, attacks.

The death penalty cases against the five men first began in 2008 under the Bush administration and was abandoned by the Obama administration for a planned trial in federal court in New York. That effort collapsed, and the prosecution was returned to the military in 2011.

Also on trial at Guantanamo is Abd al-Rahim al-Nashiri, a Saudi of Yemeni descent who also was held by the CIA. He could receive the death penalty for his alleged role in the 2000 bombing of the USS Cole in Yemen.

“If the government does provide these photos to the defense — which is still an ‘if’ at this point — it would be better late than never,” said Air Force Capt. Michael Schwartz, a military attorney for Waleed bin Attash, one of the five defendants in the 9/11 military commission. “Receiving important discovery like this so far into the case is going to further delay this trial.”

The electronic images depict external and internal shots of facilities where the CIA held ­al-Qaeda suspects after 9/11, but they do not show detainee interrogations, including the torture of some suspects who were subjected to waterboarding and other brutal techniques.

They do include images of naked detainees during transport, according to the officials, who spoke on the condition of anonymity because the material remains classified.

The pictures also show CIA personnel and members of foreign intelligence services, as well as psychologists Bruce Jessen and James Mitchell, among the architects of the interrogation program.

The government will probably resist any attempt to turn over photos of CIA officers and other personnel, arguing that it could endanger their personal security, while the defense will want to identify potential witnesses.

Brig. Gen. Mark Martins, the chief military prosecutor, declined to comment on the photos, but he said in February that his team was scouring the full, classified 6,700-page Senate report.

Lt. Col. Myles B. Caggins III, Defense Department spokesman for Military Commissions, said in an e-mail: “The prosecution is making progress in reviewing the full report in order to comply with discovery obligations required by law.”

Military prosecutors began reviewing the report earlier this year in a secure location at the Hart Senate Office Building. Officials said prosecutors learned there were more pictures available than those contained in the full report, which includes mostly mug shots of detainees.

The report’s declassified executive summary makes reference only to a picture of a wooden waterboard in Afghanistan and detainees being transported to black sites.

It’s unclear whether the military prosecutors have been able to review all the photographs and why they hadn’t unearthed them years earlier. Former U.S. officials said Martins’s team was supposed to have the same access as Senate investigators and federal prosecutors to shared electronic drives containing agency documents at a secret location in Virginia.

“It raises the question whether the agency is being cooperative with the prosecutors,” said James Harrington, the civilian attorney for 9/11 defendant Ramzi Binalshibh. “It’s beyond preposterous.”

The CIA declined to comment.

Among the images are those of cells and bathrooms at the detention sites, including a facility in Afghanistan known as “Salt Pit,” where the waterboard was photographed.

A U.S. official described the photographs of the Salt Pit as looking like a dungeon. The official added that many of the pictures appear to have been taken for budgetary reasons to document how money was being spent.

The bulk of the photographs depict black sites in Thailand, Afghanistan and Poland. There are fewer shots of prisons in Romania and Lithuania, which were among the last to be used before they were closed in 2006.

A former intelligence official who reviewed some of the photographs of the prison in Thailand described them as nondescript.

“It looked like a prison,” the former official said. “It all looked acceptable.”

A U.S. official said there are also photographs of confinement boxes where detainees such as Abu Zubaydah, who is now at Guantanamo, were forced into for hours.

Also among the photographs are images of Zubaydah shortly after he was captured in 2002; he was wounded in the leg during a shootout with Pakistani security forces. The pictures show his injury.

Later shots show him wearing an eye patch. A former CIA official said Zubaydah had a preexisting eye injury that was infected when the agency captured him. The eye was later removed.

“Why is it we are still learning about this stuff?” said Joe Margulies, Zubaydah’s attorney. “Who knows what is still out there? What else is there? That’s what is appalling.”

James Connell, defense attorney for Ammar al-Baluchi, one of the 9/11 defendants, filed a motion in January 2013 to compel production of “documents and information” relating to where the “accused or a potential witness have been confined.”

Connell said the military judge overseeing the case hasn’t ruled on that motion.

“If pictures from black sites exist, they are crime scene photographs,” Connell said. “The military commission rules require the prosecution to turn them over to the defense, but federal and international prosecutors should also get a copy — not to mention the public.”

Julie Tate contributed to this report.