Images of dead bodies in Syrian prisons, taken by a Syrian government photographer, are displayed at the United Nations on March 10. The photographer, who goes by the pseudonym Caesar, took the pictures between 2011, when the Syrian uprising began, and 2013, when he fled the country. His photos will be on display at the U.S. Capitol on Wednesday.

Lucas Jackson/Reuters/Landov

A Syrian forensic photographer, who now uses the pseudonym Caesar, documented the death of thousands of detainees in Syria's brutal prison system. He made more than 55,000 high-resolution images before he fled the country, fearing for his safety, in 2013.

He spoke publicly for the first time in July 2014, when he appeared before the U.S. House Foreign Affairs Committee, wearing a blue jacket with a hood to protect his identity.

Dozens of Caesar's photographs will be displayed again in the halls of Congress on Wednesday.

The exhibition is sponsored by the United States Holocaust Memorial Museum in cooperation with the House Foreign Affairs Committee and Senate Foreign Relations Committee.

The graphic images show beaten and bruised bodies, many are skeletal, most with signs of torture. Now, Syrian families are searching the photos online after Syrian opposition groups posted more than 6,000 images in March.

NPR spoke to a member of the group that posted the pictures, as well as a friend who identified a victim, and a lawyer working on a war crimes case. Here are their stories:

Amer fled Damascus two days after his friend, Kutabia, a 40-year-old father of two, was seized by government agents from a bookstore in Damascus. The two friends demonstrated together through 2011. They took even larger risks together, smuggling money and medicine into restive neighborhoods besieged by the Syrian government.

"They invaded on New Year's Eve at 6 p.m. I was at a café nearby. And when I finished I said, 'Let's go and say hi [to Kutaiba].' I knocked and there's no one. No lights inside. And I continued home and that's when I heard that my friend was taken to the detention center or the torture center.

This is where the story begins. Once your friend is detained by the government, you try to figure out where he is.

His parents started to ask. They usually go to people in the intelligence service and the guy will say, 'I can't tell you, you have to give me money.'

For two and a half years his parents are paying money. Sometimes they take a couple of thousand dollars and [they] get back and say, 'He's alive and well, and says hi to his sons.'"

In March 2015, Syrian opposition groups published 6,000 of Caesar's photos online. For the first time, Syrian friends and families could search the gruesome photo gallery and identify the victims. Kutaiba's picture was among the dead, killed within a month of his arrest.

Rep. Ed Royce (center), R-Calif., speaks during a July 2014 hearing of the House Foreign Affairs Committee, with "Caesar," a Syrian army defector who wore a blue, hooded jacket to protect his identity. Caesar smuggled out of Syria more than 55,000 photographs that document the torture and killings in Syrian prisons.

Alex Wong/Getty Images

"It was him. His eyes were closed. He had stitches on his forehead like the ones you see in horror movies. And I was shocked. I was in the middle of work. I don't know what sort of people can do this harm and torture to another person.

I saw his color, and was like, 'Thank God he wasn't starved to death.' He didn't have his ears taken off or his nose. So, I thought he made them furious enough to kill him right away rather than being tortured on a daily basis. It's always better to know, is he alive or is he dead."

Dr. Mohammed Ayash works with the Syrian Association for Missing and Detainees of Conscience, based in Istanbul. In March, the group published some of the Caesar photos online and has organized private showings in the Zaatari refugee camp in Jordan and even in some rebel-held neighborhoods near Damascus. Dr. Ayash is scanning all the images and documenting the dead. It is a grim task made harder still because Dr. Ayash has seen friends and neighbors among the dead.

"We have about 6,700 victims in this website. I am a doctor, I'm not a pathologist, but I describe what I see in every picture. We have children in these pictures. There are older men. There is one woman.

It comes to my dreams sometimes, because of the horrible methods. By torture, by starvation and eye gouging, and it's very hard for me.

(The work) is very important because we need the families as witnesses in any court. We need families to say, 'Yes, this is my father and my brother,' and they were taken and killed by the Assad regime."

Muna Jundi, an attorney in Flint, Mich., works with United for a Free Syria, a coalition of Syrian-American nonprofit organizations. She was part of a team of Syrians who got Caesar to testify to Congress last year and will be in Washington for the event on Wednesday. She took part in the decision to publish the photos online.

"It was systematic, the regime was using it as a way to quell the revolution.

There's a lot of missing Syrian people and a lot of people don't know the fate of their family members. They hear about it through rumors. They pay money to try and find information and really there's nothing concrete. And unfortunately there's nothing more concrete than pictures of dead bodies. So the idea was to open up to help people identify their own family members.

For an American audience, I think it was shocking. But the sheer ... mass production of this, I think, is what overwhelms. They've documented it in such detail.

Syrians inside Syria that had any experience with intelligence [services] automatically knew why the documentation had to happen.

When there's an order from above, they need evidence that those orders are being carried out. In a highly corrupt government, where you can pay people to release people, they need the evidence. They needed to keep the evidence to show that you told us this is what we need to do, and therefore, this is what we are doing."