The Syrian defector known as “Caesar,” who documented and then fled with evidence of Syrian President Bashar al-Assad’s torture and murder of more than 11,000 civilians in custody, has begun a visit to Washington, D.C., aimed at verifying and working to expose the atrocities and war crimes of the Syrian regime.

The former military police photographer, who earlier this year turned over 28,000 images of torture and murder by the Syrian military to U.S. government investigators, met Monday with State Department Ambassador-at-Large for War Crimes Stephen Rapp, members of the Syrian opposition, and officials from the U.S. Holocaust Museum in Washington. He has a series of meetings planned with U.S. officials and others throughout the week as part of a visit that was in the works for months and was a closely guarded secret until now.

Caesar’s trip is being managed by the Coalition for a Democratic Syria, a Syrian-American umbrella organization that coordinates nonprofit groups in Washington assisting the Syrian opposition.

“We are incredibly grateful to the Holocaust Museum for hosting Caesar,” said Muna Jondy, CDS’s government relations chair. “Their staff’s expertise in dealing with these kind of photographs is unmatched in the world. Further, the Holocaust Museum is an appropriate and somber setting for evidence of such monumental crimes to be highlighted and further examined.”

One of only a handful of photographers tasked with meticulously documenting the Syrian Arab Army’s killing of civilians in custody, Caesar escaped last year with more than 55,000 images showing the torture and murder of more than 11,000 men, women, and children in the Damascus area between 2011 and 2013.

The U.S. government is now reviewing 28,000 of the images and the detailed files that accompany them to determine their validity and prepare the evidence for future use in legal cases against Syrian officials, including Assad, for war crimes and crimes against humanity.

Rapp, who has already seen many of the photos, said last month that the systematic nature of Assad’s “machinery of death” was the worst he had seen since the Nazi regime perpetrated an attempted genocide of Jews and other non-Aryans during the Holocaust.

“This is solid evidence of the kind of machinery of cruel death that we haven’t seen frankly since the Nazis,” he said.

The U.S. government is nearly finished with its own forensic analysis of the photos, and Rapp said they not only appear to be genuine but that they also show a level of systematic atrocities that implicates Syrian officials, including Assad, in crimes against humanity.

“Thus far the indication is that it would be impossible to have fabricated this kind of material, and having personally seen hundreds of the images of twisted bodies with real wounds and real human beings of every shape and size, this is not phony evidence,” Rapp said. “These bodies were brought to one location from 24 other facilities in which they had been tortured to death in a variety of ways: ligature strangulation, burning, bruising, starvation, evisceration—the most horrendous things you can imagine.”

International war crimes scholar Cherif Bassiouni—who led the U.N. investigations into war crimes in Yugoslavia, Bahrain, and Libya, and helped create the International Criminal Court—also saw many of the photos of atrocities brought out of Syria by Caesar. Bassiouni told The Daily Beast last month that not only were Assad and his cohorts guilty of crimes against humanity, but top officials from countries directly aiding the Syrian army also could be implicated.

In particular, he said the torture and killing was done in a systematic way he recognized from his time studying the Soviet Union.

“What I see in the pictures is to a large extent an anomaly to the culture of the Syrian army,” he said. “The way these pictures were taken show a great deal of systematicity, reflecting a culture that is systematic in its approach. This culture, in my opinion, is more reflected in Russia.”

Caesar’s visit and the publicity it brings could push the Obama administration to confront more directly the issue of Assad’s atrocities. The Wall Street Journal reported that some administration officials have been resisting acknowledging the war crimes because doing so could hurt efforts to negotiate with Assad for an end to the conflict.

“For the administration, it is a double-edged sword,” Frederic Hof, who served as a top Obama administration adviser on Syria, told the Journal. “On the one hand, it’s going to illustrate perhaps better than anything heretofore the absolute horror of what’s going on. On the other side, it raises the inevitable question: What are we actually doing about it?”