By Laura Smith-Spark
BBC News, Washington
At first glance, the photographs seem innocuous enough. Men and women in uniform lie back in deckchairs, listen to accordion music, decorate a Christmas tree.
It seems like a carefree life - but the pictures were taken at the Auschwitz death camp at the height of the Holocaust.
The happy men and women are Nazi officials enjoying time off from the business of genocide, their images collected by Karl Hoecker, an adjutant to the camp commander.
His unique album of 116 photographs was found in Frankfurt in 1946 by a US intelligence officer, who kept it to himself for six decades before showing it to the US Holocaust Memorial Museum last year.
Museum archivist Rebecca Erbelding, who has helped to put album online, believes the very ordinariness of the scenes captured is what makes them so chilling.
"It shouldn't have surprised us that this was how they lived in Auschwitz, that this was how they unwound after a 'hard day's work'," she told the BBC News website.
"But I think it's shocking because it's a reminder that they were human beings, that they weren't red-eyed monsters, that they had pets and children and lives, and yet could do this to other people."
The find has significantly increased the number of photographs available to historians of Auschwitz-Birkenau before its liberation in January 1945.
Previously, only about 320 images were known, many of them in the so-called Auschwitz Album, which shows the arrival of Hungarian Jews at the camp in May 1944.
Hoecker's album includes the only known pictures of Dr Josef Mengele - notorious for the medical experiments he conducted on Auschwitz inmates - taken within the camp's confines.
Not a single prisoner appears in any of the images.
Ms Erbelding says the album seems to have been created very much as a personal keepsake.
The photographs show SS officers but no Auschwitz prisoners
Many of the pictures were taken at Solahuette, a little-known SS resort near Auschwitz where the camp's guards were periodically sent as a reward for hard work.
Hoecker himself is a regular fixture, decorating a Christmas tree in one photograph, going hunting or playing with his dog in others.
A series of images dated 22 July 1944 shows him eating blueberries with a group of female SS auxiliaries, one of whom pretends to cry as she holds her now-empty bowl upside-down.
On that same day, the museum's researchers found, 150 prisoners, Jews and non-Jews, arrived on a transport to Auschwitz. The SS selected 21 men and 12 women for work and killed the rest in the gas chambers.
Another image shows Hoecker enjoying a sing-along with senior SS officers, identified by the museum as Dr Mengele, former Auschwitz Kommandant Rudolf Hoess, gas chamber supervisor Otto Moll and Birkenau Kommandant Josef Kramer.
Despite Hoecker's apparent closeness to those in charge, he is a relatively little-known figure in Nazi history.
A page is devoted to Hoecker and SS women eating blueberries
After World War II, he went into banking and was only tracked down by Nazi hunters in 1961.
He faced charges at the Frankfurt Auschwitz Trial in 1963 but prosecutors were unable to find any witnesses or evidence directly linking him to the killings at Auschwitz.
Hoecker protested he was innocent, yet the photographs in his album show him socialising with those most closely involved. "It strains credulity to suggest he would have been unaware of their crimes," is the museum's verdict.
He was sentenced to seven years in prison and released on parole in 1970. He returned to his banking job and eventually died in Germany in 2000, aged 88.
Dr Stephen Feinstein, director of the Center for Holocaust and Genocide Studies at the University of Minnesota, said the photographs reveal the disconnect between the people and their actions.
"These people were trying to have a normal life while they were killing people," he said.
"What it says to me is that anybody is capable of committing genocide under the right circumstances, and what we have to figure out is what makes that possible."
He draws a parallel between what happened in Nazi Germany and more recent events in Rwanda and Bosnia, where people who had lived side-by-side for years ended up killing each other.
He also sees an echo in the scandal at Abu Ghraib prison in Iraq, where American men and women "raised as good citizens" were guilty of abusing Iraqi prisoners in their charge.
But Dr Robert Rozett, director of the libraries at the Yad Vashem Holocaust Centre in Israel, said this is not a surprise to those familiar with the history of the Holocaust.
Yad Vashem's own archives include a visitor's book kept by camp commandant Rudolf Hoess, where guests thank him for their lovely stay, he pointed out.
"What the pictures are is a very graphic visual illustration of what was going on there, and so are very important in helping us understand who the murderers were, that they often saw themselves as ordinary human beings.
"And in some ways they were - but they had an ideology of hate and engaged in mass murder.
"But they divorced themselves from that, would go home, listen to classical music, host their friends, have a good time."
For the time being, the US Holocaust Memorial Museum intends only to show the photographs online.
This means more people can access them, allows all the images from the double-sided pages to be displayed and has made the process much quicker than if they had been incorporated into the museum's physical collection, said Ms Erbelding.
"We were concerned about how people would take them because, if you take it out of context, it very much seems like a vacation album, of fun times he had," she said.
"We were very careful to include that context and it seems that most people are understanding the point we were trying to make - that the perpetrators were human beings and that it wasn't a nameless, faceless people who did this."
A lot of Holocaust survivors find the album itself and the fact it was ever created very offensive, she said, but even so "understand how important it is in terms of a reminder".