Lisi Niesner / Reuters

Survivor Petro Mischtschuk, 87, from Ukraine, wears his old prisoner's garb as he stands near the memorial site of the Little Camp at Buchenwald.

Between July 1937 and April 1945, the Nazis imprisoned a quarter of a million people in the Buchenwald concentration camp, located near the German city of Weimar. Around 56,000 of them were killed before the camp was liberated by U.S troops on April 11, 1945.

68 years later, Reuters photographer Lisi Niesner interviewed some of the remaining survivors as they returned to Buchenwald to mark the anniversary of the liberation.

Lisi Niesner / Reuters

Victor Karpus, 88, from Ukraine, stood at the muster ground where inmates gathered at dawn each day for a roll call. Karpus was imprisoned in several camps including Buchenwald for a total of three years. He even once managed to escape from a camp but got captured and taken to Buchenwald, where he remained until its liberation.

"Work or die – it was impossible to get out from Buchenwald," Karpus says.

Lisi Niesner / Reuters

Lisi Niesner / Reuters

"To each his own": An inscription on Buchenwald's iron gate.

Eva Pusztai, 88, from Hungary, sat in a wheelchair in front of a reconstructed gallows. In July 1944 she was deported to Birkenau and six weeks later to Muenchmuehle, one of 136 satellite camps of Buchenwald.

The forced labor in the arms industry or the camp's stone quarry took the imprisoned to the brink of their physical abilities. "You got just enough food to survive. I lost a third of my weight and I was almost starving to death," she says. 

"The employable have to be destroyed by work," she says, explaining the attitude of the Nazis to their prisoners. Her right eye filled up with a single tear that ran down her cheek, then she composed herself and smiled.

Lisi Niesner / Reuters

"Where is your god? Why he does not help you?" Jakob Silberstein, born in Poland in 1924, remembers the mocking of a high-level Nazi on Yom Kippur. He survived six years of captivity in Buchenwald and Auschwitz and witnessed brutal actions by the SS, being locked in a standing cubicle for a week, carrying stones and drinking rainwater for days. 

He was standing inside the gas chamber at Birkenau when an SS man asked if any of the men were skilled laborers. "I stated I was an electrician, which luckily saved my life," he said. After the liberation he found out that none of his family or friends had survived the war. He now lives in Israel and tirelessly tells his story.

Lisi Niesner / Reuters

Lisi Niesner / Reuters

Urns are displayed in a room adjacent to the crematorium at Buchenwald.

Professor Elling Kvamme, 94, from Norway, stood at the site of Barrack Block 22. He was teaching medicine at a university in Oslo in 1943 when he was arrested for his connections with underground politics. "Students are always dangerous and the Nazis realized it very quickly," he explained.

He was forced to take part in the Nazi program of Germanization and had to work at the pathological facility in Buchenwald. Before the dead were cremated in an incineration system developed to veil the traces of murder, specimens were taken from their corpses for anatomical collections.

Lisi Niesner / Reuters

Vasile Nussbaum, 83, from Romania, spent a year in Auschwitz and Buchenwald. "Buchenwald was a sanatorium in comparison to Auschwitz" he recalls without hesitation.

Nussbaum revisits the site of the camp every year on liberation day. "You never know what’s coming, today we are 83 years old and in the next year we are no more here", he says.

Lisi Niesner / Reuters

Barracks behind trees at Buchenwald.

Editor's note: Pictures taken between April 11-14, 2013 and made available to NBC News today. Read more at Reuters' Photographers Blog.

Courtesy Auschwitz Jewish Center

The Chevra Lomdei Mishnayot synagogue (c. 1939-1941).

REGENSBURG, Germany -- A synagogue near the former Nazi death camp Auschwitz is on the verge of collapse, officials warned on Wednesday.

The head of the Auschwitz Jewish Center Foundation, which maintains the historic building in the southern Polish city of Oswiecim, said in a phone interview that the synagogue is on unstable ground and if it is not reinforced soon, it may crumble.

"There are already small cracks visible," Tomasz Kuncewicz said. "A thorough examination found that the ground is unstable and with heavy rain or something similar, anything can happen."

If the Chevra Lomdei Mishnayot Synagogue were to collapse, the only surviving Jewish house of prayer in the city would be ruined.

Oswiecim, once an ordinary town home to a large Jewish community, became an international symbol of the Holocaust when Nazi Germany ran its largest and deadliest concentration camp just two miles from the city center during World War II. Some 1.5 million people, mostly Jews, were killed at the Auschwitz-Birkenau camps.

"Several synagogues were located in the area, and this was the only one not destroyed by the Nazis," Kuncewicz said.

Jacek Bednarczyk / EPA

Students visit the Chewra Lomdei Mishnayot Synagogue in Auschwitz, Poland, on Wednesday.

The synagogue itself seems to trace the tragic path of the Jewish community in the area.

Built around 1913, it thrived until the Nazi occupation. During World War II, the interior was gutted and it was used to store ammunition.

After the war and the liberation of the concentration camp, a group of Jewish survivors restored the building provisionally, but it stopped operating when the small group emigrated from Poland shortly thereafter. In the 1970s, the country's communist government nationalized the building and turned it into a carpet warehouse.

It wasn't until 1998 that the synagogue was turned back over to the Jewish community, a historic first in Poland after the fall of the communist regime in 1989. It was rededicated in 2000 in an effort to rekindle the Jewish community that had been so vibrant in the city decades before.

Today, it is not only a place of prayer, but also a historical site and educational center that draws 25,000 visitors each year.

Organizers are seeking $300,000 for the renovation effort, the majority from donations, but they also are asking for help from government agencies.

Kuncewicz said he hoped to start the repairs this spring: "We are working very hard to raise money for this project, to make sure the synagogue will stand."