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.