The regime of Syrian President Bashar al Assad is holding 150,000 civilians in custody, all of whom are at risk of being tortured or killed by the state, the Syrian defector known as “Caesar” told Congress on Thursday.

According to a senior State Department official, his department initially asked to keep this hearing—in which Caesar displayed new photos from his trove of 55,000 images showing the torture, starvation, and death of over 11,000 civilians—closed to the public, out of concerns for the safety of the defector and his family. Caesar smuggled the pictures out of Syria when he fled last year in fear for his life. Caesar’s trip had been in the works for months.

(Warning Graphic): Photos of Assad's Killing Machine

There was no audio or video recording allowed at the hearing; the House Foreign Affairs Committee said that decision was made in consideration of Caesar’s safety. He sat at the witness table disguised in a baseball cap and sunglasses, with a blue hoodie over his head. “We recommended to Congress a format for today’s briefing that would have allowed press access while addressing any security concerns,” said Edgar Vasquez, a State Department spokesman. A committee staffer alleged State had tried to prevent the hearing from happening at all.

The packed committee room sat in silent horror as new examples of Assad’s atrocities were splashed on the large television screens on the wall and displayed on large posterboards littered throughout the hearing room. Caesar spoke softly to his translator, Mouaz Moustafa, the executive director of the Syrian American Task Force, a Washington-based organization that works with both the Syrian opposition and the U.S. State Department.

“I am not a politician and I don’t like politics,” Caesar said through his translator. “I have come to you honorable Congress to give you a message from the people of Syria… What is going on in Syria is a genocidal massacre that is being led by the worst of all the terrorists, Bashar al Assad.”

The international community must do something now or the 150,000 civilians still held in regime custody could meet the same bleak fate, Caesar said. America had been known as a country that protected civilians from atrocities, he argued, referring to past humanitarian crises such as ethnic cleansing in Yugoslavia.

“These bodies that we have [in the photos]… no one here can bring their life back to them, but I am here to tell you there is more than 150,000 people still incarcerated in the jails of Bashar al Assad and their fate will be the same fate as those who I have taken pictures of,” he said. That figure could not be independently verified.

Caesar told the committee members his story. After spending two years meticulously documenting the systematic torture and murder of thousands of men, women, and children, he carefully planned his escape with the photos and the files that accompany them. The FBI is near complete in its effort to verify them, increasing their evidentiary value for future war crimes prosecutions.

“I saw pictures of young children and the very elderly as well, and pictures of women. Sometimes I would come across the pictures of some of my own neighbors and people that I recognized. I was horrified but I would not tell them the fate of their children,” out of fear of the regime’s retaliation, Caesar sad. “My religion did not allow me to be quiet about these horrendous crimes that I have seen.”

International war crimes scholar Cherif Bassiouni—who helped create the International Criminal Court—testified that the Caesar photos reminded him of the systematic killing and documenting of said killing devised and used by the Soviet Union and the KGB.

“The Russian system, the KGB system, was different but is exactly the same as the one the Syrian regime has adopted,” he said. “If there is Russian involvement [in Assad’s atrocities]… than there is responsibility through the chain of command.”

International war crimes prosecutor David Crane, who led the first large research project looking at the Caesar photos, said that the atrocities evoked memories of the Holocaust, a sentiment expressed last month by the State Department’s Ambassador-at-Large for War Crimes, Stephen Rapp.

“We rarely get smoking gun evidence in my business… but what we found was just that,” said Crane. “The photos show crimes the likes of which we have not seen since Auschwitz.”

Frederic Hof, a former State Department senior official dealing with Syria, testified that the photos should compel the Obama administration not to work with the Assad regime, a plan some senior Obama administration officials are considering in light of the growing threat in Syria posed by extremist groups such as ISIS.

“This briefing eliminates the moral admissibility of any collaboration with the Assad regime,” said Hof, adding that the only other plausible option was to drastically increase American support to the Free Syrian Army.

The Obama administration has proposed a new $500 million program to train and equip the FSA, but members of Congress have been complaining that the administration hasn’t provided any details and doesn’t seem to be pushing for the funding on Capitol Hill.

At the hearing, members of both parties called out the administration for not doing enough to confront the ongoing crimes against humanity in Syria.

Committee Chairman Ed Royce called for a tribunal to be established to hold the Assad regime accountable for war crimes. “Pure and simple, these photos cry out for justice,” he said.

“We want to look away, but we must not,” said ranking Democrat Elliot Engel. “This is happening right now in Syria as we speak, and we can do more to stop it.”