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.”