(Photo illustration: Yahoo News. Photos: Remy de la Mauviniere/AP; Courtesy of The Caesar Team/Coalition for a Democratic Syria)

A yearlong analysis by the FBI has concluded that controversial photographs showing the torture of Syrian political prisoners are authentic, providing powerful new evidence to support charges of extensive human rights violations by the regime of Syrian President Bashar Assad.

The photographs, smuggled out of Syria by a defector two years ago, show no evidence of being manipulated and “appear to depict real people and events,” the FBI concluded in a report compiled at the request of the State Department.

A top State Department official said the FBI report, a copy of which was obtained exclusively by Yahoo News, could provide fresh impetus for international war crimes prosecutors to bring criminal charges against top Syrian officials.

But, by refocusing attention on Syrian abuses, it could also complicate Obama administration efforts to persuade Congress to back the Iranian nuclear deal signed today in Vienna.

Iran has been a major backer of the Assad regime, and Assad himself today sent a congratulatory telegram to the country’s supreme leader, Ayatollah Ali Khamenei, saying he expected the accord to lead to more support “with greater drive.”

After more than a year of analysis by the FBI lab in Quantico, Va., the five-page report was completed last month. It focused on 242 of the grisly photos — there were more than 55,000 in all — showing emaciated, bruised and scorched bodies, some lined up in a warehouse with ribs protruding, in scenes that have been compared to images from the Nazi Holocaust.

They were taken by a former official government photographer-turned-defector who, using the codename “Caesar,” smuggled them out of Syria two years ago on thumb drives concealed in his shoes.

Related Slideshow: Newly released to Yahoo News: Syrian torture photos

When the photos first surfaced last year, Assad officials denounced them as forgeries concocted by Syrian rebel groups.

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Any expert “could easily find out that these pictures are fake and that they have no relation to prisoners or detainees in Syrian prisons,” the Syrian Justice Ministry said in a statement at the time.

But the FBI report contradicts that claim.

The bodies and scenes depicted in the 242 samples analyzed by FBI analysts “exhibit no artifacts or inconsistencies that would indicate that they have been manipulated … [or] are the result of computer-generated effects,” states the report by the FBI’s Digital Evidence Laboratory.

Some of the photos, most of which were taken on a Nikon Coolpix P50 digital camera, do show text that was added to cards seen in the photos. But these alterations “had no effect on the bodies or remainder of the scenes depicted therein. … Therefore, all 242 images appear to depict real people and events,” the report concludes.

House Foreign Affairs Committee Chairman Rep. Ed Royce, center, speaks as Syrian Army defector Caesar, in a blue hooded jacket, who smuggled out of Syria over 50,000 photographs documenting the torture and execution of more than 10,000 dissidents, listens through an interpreter during a briefing in July 2014. (Photo: Alex Wong/Getty)

The FBI report is “very significant,” said Mouaz Moustafa, the executive director of the Syrian Emergency Task Force, a Washington-based group that helped smuggle the photos out of Syria. “It verifies that the Syrian regime is engaged in genocidal massacres in its jails. These are still going on today — and we’re not doing anything about it.” Working with a team of international lawyers and human rights advocates funded in part by the government of Qatar, the group turned the images over to the U.S. State Department last year.

Stephen Rapp, the State Department ambassador for global criminal justice, said Tuesday that the FBI report has already been shared with officials at a United Nations commission investigating Syrian human rights abuses, as well as with war crimes prosecutors from European countries looking for evidence that some of their citizens may have been tortured inside Syrian jails.

The report “has increased the possibility that there will be prosecutions of the responsible officers of Syrian torture centers in the courts of states where victims had citizenship or their families are in refuge,” Rapp said in a statement emailed to Yahoo News.

The FBI conducted its analysis at Rapp’s request. The bureau informally told Rapp last year that it had seen no evidence that the photos had been doctored, but the written report, for which some members of Congress had been pressing, is the first definitive conclusion. Portions of the report, possibly discussing efforts to identify the individuals shown, were redacted in the copy obtained by Yahoo News.

The report comes at a crucial moment in the debate over U.S. policy toward Syria. Some members of Congress are pressing for increased aid to moderate Syrian rebel groups and the creation of no-fly and protected zones to shield civilians from air attacks, which have included deadly barrel bombs containing poisonous chlorine gas.

They argue that while the brutality of the Islamic State has been on ample display through its release of videos of beheadings, the far more extensive atrocities of the Assad regime have been less visible and have received much less public attention.

Those members — including House Foreign Affairs Committee Chairman Rep. Ed Royce, R-Calif., and ranking minority member Rep. Eliot Engel, D-N.Y. — plan to press the issue at an informal House-Senate hearing on Wednesday that will display the Caesar photos. The meeting, which will be held in the Rayburn House Office Building, is being co-sponsored by the U.S. Holocaust Memorial Museum.

Royce, who last year had arranged for Caesar to testify before his panel with his face concealed by a hood, said today that he was “pleased but not surprised that these photos of sheer brutality have been authenticated. … A year later, these photos demand action.”

Also slated to attend the hearing are Senate Foreign Relations Committee Chairman Sen. Bob Corker, R-Tenn., and ranking Democratic member of the committee Sen. Ben Cardin, D-Md.

The hearing will also include testimony from Qutaiba Idelbi, a 25-year-old Syrian human rights activist who was arrested and taken to a military prison after participating in protests against the regime during the 2011 Arab Spring. He told Yahoo News that he plans to describe how he was tortured inside the prison: lashed with whips, stomped on the head, doused with cold water and squeezed between wooden planks in a procedure known as the “flying carpet” that left him “screaming with pain.”

Idelbi said he recognizes at least four of his friends among the prisoners in the cache of photos. “I don’t want anyone to see what I saw or live what I lived,” he said. “This is the goal of my life.”