A Syrian defector who smuggled out thousands of photos of mutilated corpses, showed some of those images in Washington on Monday and said they depicted prisoners who were tortured and killed by the security services of Syrian President Bashar al-Assad.

The presentation to a small group of reporters and researchers at the U.S. Holocaust Memorial Museum marked the first time that the defector, identified only as Caesar, has appeared publicly to answer questions about a trove of materials seen by human rights organizations as evidence of war crimes committed by Assad.

The photographs show as many as 11,000 victims’ bodies — many of them tagged as they lie in makeshift morgues — and were turned over to United Nations and FBI investigators after the defector fled Syria last year. The photos have been described as authentic by U.S. officials.

The defector said that he and other government employees took the images while working in jobs that required them to photograph the dead for the Assad government’s records.

The photographs presented Monday showed dozens of badly mutilated and emaciated corpses, many of them disfigured by beatings, missing chunks of skin from lashings, or bearing rashes that experts said may reflect exposure to toxic substances.

Despite their potential value as evidence, the defector said that Assad officials required that the photos be taken as proof of the deaths and as proof that the orders of leaders of the government’s security agencies were being carried out.

The defector spoke through a translator.

The existence of the photos has been previously reported. They were viewed by U.N. Security Council officials in April as part of an unsuccessful effort by France to refer Syria to the International Criminal Court.

The defector has held a series of meetings with U.S. government and congressional officials in Washington this week, according to Mouaz Moustafa, executive director of the Syrian Emergency Task Force, who presided over a question-and-answer session at the museum on Monday.

Michael Chertoff, the former secretary of the Department of Homeland Security and a member of the museum’s governing council, attended and introduced the defector.

Wearing sunglasses and a San Francisco Giants’ baseball cap pulled low over his face, the defector spoke on the condition that he not be photographed or recorded. He said that his job as a photographer for the nation’s military morphed during the Syrian civil war into an assignment recording images of the bodies being brought to Hospital 601 in Damascus.

“My conscience would not allow me to continue taking pictures and not be able to leak them out or share them,” Caesar said. “It was horrendous.” He said he began leaking the images to Syrian opposition groups in 2011.

At first, he said there were only five or 10 a day, but the numbers swelled to 50 or 60 as the conflict grew. He said the photos were taken between 2011 and 2013. Those shown Monday as part of a video presentation showed the corpses of men whose eyes had been gouged out, and dozens of bodies starved to skin and bones.

All were held at Syrian prisons, the defector said, and many were marked with number sequences on their torsos, or on tags attached to their heads or bodies. In some cases, families were given death certificates saying they had died of heart attacks or other natural causes. The Syrian government has rejected the claims.

The photos have not been released publicly. But Moustafa on Monday said he would seek to work with the Holocaust Museum to post some online.