The State Department has obtained 27,000 photographs showing the emaciated, bruised and burned bodies of Syrian torture victims — gruesome images that a top official told Yahoo News constitute "smoking gun" evidence that can be used to bring war-crimes charges against the regime of Syrian President Bashar Assad.

The photos are "horrific — some of them put you in visceral pain," said Stephen J. Rapp, the U.S. ambassador-at-large for war crimes, in an interview. "This is some of the strongest evidence we've seen in the area of proof of the commission of mass atrocities."

The photos — a small number of which will be put on public display for the first time on Wednesday at the U.S. Holocaust Museum — were smuggled out of Syria by an official regime photographer who has since defected and is known only by his code name, Caesar.

They were shown at a closed-door session of the House Foreign Affairs Committee in July where Caesar, wearing a hood, testified. They are now being analyzed at Rapp's request by the FBI in part as an effort to determine whether any U.S. citizens may have been among the victims — a finding that could be the basis to bring criminal charges in the U.S. against officials of the Assad regime.

The Syrian government has officially denounced the photos as fakes and suggested many of the corpses seen are actually of militants who died in battle.

While FBI agents are still reviewing the photos, Rapp said that bureau officials have already "informally" told him "they think it is impossible they could be forgeries. There is no evidence of doctoring."

(A bureau spokesman confirmed only the review of the photos, adding: "It will take some time to complete the authentication process.")

Syrian Army defector Caesar, (in a blue hooded jacket) who has smuggled out of Syria more than 50,000 photographs that document the torture and execution of more than 10,000 dissidents, listens to an interpreter during a briefing before House Foreign Affairs Committee July 31, 2014 on Capitol Hill in Washington, DC. (Alex Wong/Getty Images)

The story behind the photos begins in March 2011, when Arab Spring protests against the Assad government swept through Syria. As the military began rounding up suspected dissidents, Caesar — a military police officer — was assigned to lead a team of 11 photographers whose job it was to document the deaths of detainees brought to a military hospital from three detention centers around Damascus.

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But by the summer of 2013, Caesar has told investigators, he was so sickened by what he was seeing that he made contact with Syrian rebels. "I can't do this anymore," he told them, according to David Crane, a former war-crimes prosecutor for Sierra Leone who spent hours interviewing Caesar as part of a separate review of the photos commissioned by the government of Qatar.

Caesar began smuggling his photos to the rebels, providing them with thumb drives concealed in his shoes, Crane said. To protect his family, Caesar faked his death, staging an elaborate funeral, before he escaped from Syria in August 2013. He is now in hiding in Europe.

The photos, according to Crane, document "an industrial killing machine not seen since the Holocaust." They show corpses, some of them lined up in a warehouse, many appearing to be victims of starvation, their ribs protruding from emaciated bodies.

Some show men whose eyes were gouged out; others had bruises and lacerations consistent with beatings and in some cases strangulation, according to a report that Crane co-wrote about the photos released in January.

Rep. Ed Royce, R-Calif., the chairman of the House Foreign Affairs Committee who had called Caesar as a witness at the closed-door hearing in July, said that when he first saw the photos he thought of his father. As a member of Gen. George Patton’s Third Army, “my father had taken photos at Dachau when it was liberated, of the bodies stacked up at the ovens. This is eerily reminiscent. It's absolutely appalling.”

What's also noteworthy about the photos, according to Crane, was the methodical nature of the enterprise: Each photo includes tags with numbers and letters that identify each of the victims as well as the detention center where they were imprisoned. One purpose: so military officials who ordered their deaths could have proof "their orders were carried out," said Crane.

Crane — who, as a war-crimes prosecutor for an international tribunal, brought the indictment against former Liberian President Charles Taylor — originally reviewed the photos along with two other international war-crimes prosecutors on behalf of a London law firm hired by the Qatari government.

He then presented the photos for two hours at a session of the U.N. Security Council in support of a French-sponsored resolution authorizing an international war-crimes tribunal for Syria in April.

Syrian Army defector Caesar, (in a blue hooded jacket) who has smuggled out of Syria more than 50,000 photographs that document the torture and execution of more than 10,000 dissidents, listens to an interpreter during a briefing before House Foreign Affairs Committee July 31, 2014 on Capitol Hill in Washington, DC. (Alex Wong/Getty Images)

After his presentation was complete, Crane said, the Security Council fell silent. The U.S. ambassador, Samantha Power, "was blinking back tears," said Crane. (A spokesman for Power did not respond to a request for comment. But in a statement at the time, Power said, "Nobody who sees these images will ever be the same.")

But the French resolution was vetoed by the Russian and Chinese representatives. That has left Rapp with what he acknowledges are "jurisdictional challenges" in bringing war-crimes charges against the regime officials responsible for the dead bodies. (A finding that U.S. nationals are among the victims could help overcome some of those challenges by allowing a Justice Department prosecution in U.S. courts.)

But Rapp said he is not deterred. His office is working with an international team of investigators — under the direction of a private group called the Syria Justice and Accountability Project — to collect documents and other witness testimony that can be used to corroborate the photos. (The U.S. is also supporting a separate team of investigators developing evidence of war crimes by the Islamic State militant group.)

The U.S. government has contributed $1 million to the effort to investigate the Assad regime's abuses. And already, Rapp said, some documents showing orders to arrest particular detainees have been uncovered. Investigators are seeking to determine if those orders can be matched up with the bodies of detainees seen in the photographs.

But there is still much more work to be done. Because many of the photos had to be compressed by Caesar to get them to fit on thumb drives, crucial metadata — which would yield the precise date and time that each image was recorded — was lost. Confirming the deaths of detainees shown in the photos with family members who are still inside Syria is also a problem.

Still, Rapp said, "we are laying the foundation for the day when there will be accountability. This is the kind of evidence that can support prosecution of people all the way to the top."