After Holocaust survivor Joseph Weinberger entered a nursing home last month, his nephew began cleaning out his Borough Park apartment and discovered an old suitcase containing his naturalization papers, German currency and a book about the Nazi doctor, Josef Mengele — all relatively standard-issue belongings.

Then Yitzchok Mermelstein reached into a bag and pulled out a mundane artifact, but one with the power to drive home the reality of the Holocaust with the force of a sledgehammer. It was a bar of soap bearing the initials “RIF.”

“When I opened the bag and saw ‘RIF,’ I remembered hearing that the Nazis made Jews into soap, and I thought that maybe this was one of the bars,” recalled Mermelstein, 52, of Flatbush.

“I believe I heard about it at the Holocaust Museum in Washington and that Yad Vashem [the Holocaust museum and memorial in Israel] has a couple of bars,” he added. “I wrapped it in a towel. Seeing the Mengele pictures and realizing that my uncle used the suitcase when he came to America, I put two and two together.”

Mermelstein said he went home and looked on the Internet for information about such soap and saw a picture of a bar purported to have been produced by the Nazis from the bodies of concentration camp inmates. A picture of one of those bars of soap “looked the same as I had, only it had a different number” below the initials RIF. (The initials are believed to be those of a German soap manufacturer.)

Mermelstein said he then visited his uncle in the nursing home but that his uncle, who has dementia, was unable to remember where he got the soap or anything about it.

Mermelstein said that “since I know that human remains have to be buried,” he contacted people he knew at Misaskim, an Orthodox Brooklyn-based group that provides support to families in mourning and other services.

“They asked if it was real,” Mermelstein said. “I said I wish it wasn’t, but it is. … My mother was in a concentration camp, perhaps he got the soap from her. … It’s a one in a billion chance you find something like this.”

At first there was talk of burying the soap in a dignified ceremony. But as more and more people heard of it, questions were raised about whether the Nazis ever really did use human fat to make soap. The soap, which is brownish in color and measures about two inches by one inch, was then locked up in Misaskim’s offices for safekeeping.

A staff member at Misaskim researched the matter and posted a story about it on the group’s website. In it, she recounted how Yosef Weinberger had spent the war hiding from the Nazis in cellars and bunkers; the Nazis killed his parents and nine of his 13 siblings.

The staff member, Suri Roth, found that judges in the Nuremberg Trials accepted the testimony from someone who said he had worked at a laboratory that was involved in the production of soap that contained human fat. A bar of such soap was actually introduced as evidence in the trials. Five years ago, that soap was tested by Poland’s National Remembrance Institute, which announced that it did indeed contain human fat.

But Roth said Holocaust experts today believe that only a small amount of soap containing human fat was ever produced.

Holocaust historian Deborah Lipstadt told The Jewish Week that “there is no proof that the Nazis made Jews into soap in a mass fashion. … There were attempts, but it was never practical.”

She noted, however, that one museum, the Chamber of the Holocaust on Mt. Zion in Israel, actually displays a bar of soap purportedly made by the Nazis from human fat. The museum, maintained by the Diaspora Yeshiva, is located in a cellar and built in 1949. Lipstadt said she knows of no other museum with such a display.

The Virginia Holocaust Museum in Richmond shows footage of a Russian film that was played at the Nuremberg Trials. It shows Russian soldiers coming upon a skeleton in a vat at the Gdansk Anatomical Institute, from which it was deduced that the Nazis used human fat to make soap.

Jay Ipson, the museum’s founder, said he knows questions have been raised about the veracity of the soap story and that all he is doing is showing visitors what was presented at the Nuremberg trial.

A survivor, Ipson said that after the war many survivors had bars of this soap that they believed contained human remains. He said one survivor who spoke at his synagogue a few years ago recalled seeing survivors burying a number of bars of soap in a cemetery in Munich.

But Peter Black, senior historian at the United States Holocaust Memorial and Museum in Washington, D.C., said questions about the soap are frequently asked by visitors and that his answer is, “It didn’t happen.”

“There is nothing we can hold our hats on that would indicate the Nazis tried this even experimentally,” he said.

Asked about the Nuremberg Trials, Black said the evidence presented there “gave the rumor some legs.” In addition to the Russian film, two British prisoners of war testified that they worked at the anatomical institute, but Black said their testimony was “inconsistent.” One of them gave the court the “recipe” that was in the institute for making the soap — and there was no mention of using human remains.

Black said he read the report of the tests conducted five years ago in Warsaw, but he noted, “The forensic work was never released.”

Regarding the initials “RIF” on the soap bars, Black said it is widely believed this stood for, “Jews rest in peace.” In fact, he said, it was the initials of the soap manufacturer, Reich Center for Industrial Fat Provisioning.

“The Nazis did a lot of things that were very ghoulish, but for some reason the shock value of the soap, of leather goods and lampshades made of human products capture the imagination,” Black said.

(Mark Jacobsen’s recent book “The Lampshade” is an account of the author’s attempt to authenticate whether a lampshade made from human skin could be traced back to the Nazis.)

Black said there is no proof that such objects contain human product “and if it does, we don’t know it is from Jews. … And the difficulty is tying it back to the Nazis. There is nobody who was close enough to have seen the process. There were stories in Romania, Auschwitz and Danzig. But if you follow them to their source, there is nothing.”

Mermelstein said he would like to have the bar of soap from his uncle’s suitcase tested to see if it contains human fat. If it does, he said, it would be buried with proper respect.