This story contains descriptions and images of torture victims that will be upsetting to some readers.

Washington (CNN) He thought all he had to do was get the photographs out to the world. He'd need to risk his life time and time again. Smuggle contraband across front lines. Make meticulous plans in secret. But it would be worth it.

Surely if the world saw what he saw -- children, women and men starved and slaughtered by the Syrian government -- things would change.

So he saved the photos surreptitiously to thumb drives, smuggled them out of the country and laid bare the brutality of the Bashar al-Assad regime

The horrifying pictures -- almost 55,000 of them, taken by him and others -- showed scenes that were compared to the depravity of the Nazis.

Alleged torture of prisoners by Assad regime – A belt allegedly used for strangulation is left behind on a corpse.

Alleged torture of prisoners by Assad regime – The report alleges that this image shows marks left behind from strangulation.

In an image from a report by three renowned war crimes prosecutors, the emaciated remains of a man allegedly killed in Syrian custody are shown. CNN received the photos partially obscured to protect the identity of the source.

Alleged torture of prisoners by Assad regime – In an image from a report by three renowned war crimes prosecutors, the emaciated remains of a man allegedly killed in Syrian custody are shown. CNN received the photos partially obscured to protect the identity of the source.

A team of internationally renowned war crimes prosecutors and forensic experts has found "direct evidence" of "systematic torture and killing" by the Syrian President Bashar al-Assad's regime, the lawyers say. CNN was given these photos by the lawyers involved with the investigation, who received them from a defected Syrian military police photographer.

Alleged torture of prisoners by Assad regime – A team of internationally renowned war crimes prosecutors and forensic experts has found "direct evidence" of "systematic torture and killing" by the Syrian President Bashar al-Assad's regime, the lawyers say. CNN was given these photos by the lawyers involved with the investigation, who received them from a defected Syrian military police photographer.

But a world that said "Never Again" after the Holocaust appeared to shrug.

"I honestly thought that if I could have the courage to go for the years that I did ... endangering my life every single day, that once I came out and showed the world what I had, that the entire conscience of the world would move," he says.

But it didn't.

So the man -- known to the outside world only by his codename "Caesar" -- decided he had to risk his life again. Maybe this time would bring the action to stop the killing. He pulled up the hood of his blue parka and slipped on gloves, afraid that giving any sign of his identity to pro-Assad forces could bring death to his door, and agreed to another interview.

"I am pleading for the American people, for the United States Congress, for the American administration, to please save the Syrian people, save these people that do not deserve the hellish nightmare that they're living in," he tells CNN, using his own translator.

His return to Washington means he must face not just the risk but the horror once again. Specifically, the graphic photographs that he and others took to document the deaths of thousands of people for the Syrian government.

There is the sandy garage that looks like a slaughterhouse, except no one would treat animals that way. The emaciated bodies of fathers, mothers and children strewn on floors, many showing signs of torture. Holes where eyes were gouged out, probably while the people were still alive. That's just one photo. In others, bodies show marks that appear to be from electric cables used to beat them. Broken jaws and broken teeth. Each corpse is numbered.

Seeing the pictures again is hard for Caesar. However, it's not like the images have ever really left him. These bodies -- some of the people were his neighbors and friends -- haunt his sleep as well as his waking moments.

"There isn't a moment that passes that I don't think about them. It stays in your mind, it stays in your heart," Caesar says. "When I close my eyes, I see them still."

There was collective horror, outrage and calls for action when Caesar's photos were first revealed in 2014 to CNN and The Guardian. Now, standing at the Holocaust Museum where his photos are on display, Caesar becomes exasperated when explaining what happened next.

"The world has said the words 'Never Again' many, many times. But these have been mere words without any real meaning or action behind them," Caesar says, the frustration mounting in his voice, his hands clenching into fists.

Caesar says his photos represent two and a half years' worth of grim recordkeeping by him and his colleagues mostly in just one area of the capital, Damascus. There were likely other teams doing the same work elsewhere.

It's been six years since Caesar left. In that time, Assad's tactics have only grown more vicious. He's accused of using indiscriminate barrel bombs and chemical weapons as he takes back rebel territory.

Detention and torture also continue, according to the Syrian Network for Human Rights . The nongovernmental organization reports that the Assad regime has killed more than 14,000 by torture, including 142 people in the first half of this year. It said more than 128,000 people were still detained or "forcibly disappeared" by the government since the uprising began in 2011.

A panel of internationally renowned war crimes prosecutors and forensic experts analyzed the photos in 2014 and declared them evidence of "systematic torture and killing" by Assad's regime. They estimated they showed about 11,000 victims. One of the experts called the images "reminiscent of the pictures of those (who) were found still alive in the Nazi death camps after World War II."

The Syrian Government denied those claims and rejected the photos as fake.

But there will be no prosecution at the International Criminal Court, as Syria is not a member. France tried to bring a resolution in the United Nations Security Council to refer the case to the court, but Assad's ally Russia along with China vetoed the measure.

The only concrete action by the United Nations was to allow an exhibition of some of the photos inside its New York building.

The photos have been used as evidence to press for the prosecution of at least one individual, but the days, months and years of failure by Western governments to hold the regime accountable have made Caesar disappointed, frustrated and angry.

A woman reacts as she looks at a gruesome collection of Caesar's images at the United Nations Headquarters in New York.

"All of those people that we documented are dead. The world cannot give them their lives back," he says. "But I ask and I ask in the name of humanity of the entire world to please help save the rest." His passion is put into English by Mouaz Moustafa, Syrian Emergency Task Force Executive Director, who accompanies him around Washington.

So Caesar walks the halls of Congress, shuttling from meeting to meeting, in the blue parka that disguises his identity. He says he hopes this will be his last visit to the US capital, the last time he will have to risk leaving his family who have found refuge in a Western European country he will not name. He believes Washington holds the final, best hope to make a real difference in stopping or, at the very least, slowing Assad's brutality.

Some in Congress are listening. They even have a sanctions bill in his name. The Caesar Syrian Civilian Protection Act would slap new sanctions on Syrian leaders and would commit the US to support international prosecution of those accused of human rights abuses. The bill has passed the House of Representatives three times since 2016 with bipartisan support. Each time it has languished in the Senate, where it sits today.

Syrian defector Caesar conceals his identity as he listens with a translator to the House Foreign Affairs Committee in 2014.

Caesar may have an important new ally in Republican Sen. Lindsey Graham, an ally of President Trump, who sometimes has the President's ear and sits on the Senate Foreign Relations Committee.

"If this bill passes it would be the mother of all sanctions against the Syrian bank, the Assad regime," Graham tells CNN. "I am going to push to take whatever time on the floor we need to pass this legislation. ... I want to send a clear signal to the Syrian people that we are with you. I want to send a clear signal to the Assad regime that we are out to get you. And I want to let everybody know who's doing business with Assad, you do so at your own peril."

Graham also revealed to CNN he plans to push a new resolution declaring President Assad a war criminal in the hope that forcing his colleagues into a stark choice will break the resistance to doing more in Syria.

Caesar doesn't lay blame on any one administration or political party for US inaction. His voice gets stronger and his fists tighten once again when he describes what that inaction has signaled.

"The fact is that the Assad regime considers the silence of the world and considers the acquiescence and the fact that the United States and others have stood as bystanders to the conflict, as a green light for him to continue to slaughter innocents."

Caesar has wavered about doing this interview. He canceled two early meetings with CNN. It's clear there's so much on the line for him to do it, but too much on the line for him not to.

His nervous energy is palpable throughout. He fidgets. He has trouble catching his breath. His eyes constantly scan the room as he swallows hard. He has intense fear of cameras. As his camera allowed him to expose Assad's atrocities, he fears that a camera may now expose him.

To understand Caesar's simultaneous almost paralyzing fear and determination, you need to hear his story. It's not one Caesar enjoys telling. But it explains some of the complexity of the Syrian conflict and why he needs America to act.

Before Syria descended into civil war, Caesar was part of a military police forensic photography unit in Damascus. He photographed the aftermath of car crashes, fires, suicides and any accidents that involved the Ministry of Defense. When protests began in March 2011 as part of the Arab Spring, he and the others were reassigned.

"Our entire team was asked to go to military hospitals and take photographs ... of detainees, of civilians that have been tortured to death," Caesar explains. "And that became our sole job for the entire time until I escaped."

It started out as a few victims a day, he says. Then it might be 10 or 11. Soon, there would be days when there were 40 or 50 bodies. By September 2011, he decided he couldn't be a part of it anymore. But he wanted to expose the atrocities first. So he stayed, secretly copying the photos and planning how to get them and himself out of Syria.

"I would work for hours taking photographs, loading the photographs, the files on the state computer, categorizing them, doing the reports," Caesar says. "I would see innocent people that have been tortured in some of the most horrific ways that I have ever seen, that I couldn't even imagine."

His voice slows and breaks as he recalls struggling to hide his emotions.

"I would see such horrendous mutilation of the bodies, even if it was a relative of mine or a neighbor or a friend, I wouldn't even be able to recognize (them)," he says.

"I would have to pray that a tear does not come down my face," he says. "If they saw one tear, if they saw one expression on my face that showed sympathy, then I would be killed as (well as) my family."

There was danger outside of work, too. While Caesar worked for the regime, he lived in an area controlled by those opposing Assad.

He feared the rebels would kill him if they knew he was Syrian military police and the government would torture him to death if they caught him with the thumb drive of photos.

After two years of this, he did escape. But he says his mission is not over.

"I have had to take on a trust, a burden from countless civilians that have suffered the most horrific fates, tortured to death under the Assad regime," Caesar explains. "And so I come here and I will continue to come here year after year if I have to, until we can hold these war criminals accountable, until we can deter the killing of civilians."

He just hopes that his speaking out will bring justice and not death.

Not more deaths of Syrian civilians. Not his own -- if they find out who he is.

Caesar looks toward the White House after a meeting with administration officials in a nearby building.

With the weight of those tens of thousands of victims he photographed on his shoulders, the weight of knowing he is one of the last voices that can speak up on their behalf, he quietly shakes our hands, zips up his parka and heads off to more meetings in Washington.