Prime Minister Benjamin Netanyahu urged the Polish government on Sunday to track down the Nazi sign stolen from the entrance of the Auschwitz death camp memorial.

The sign, stolen on Friday from the death camp in Poland, has the infamous saying Arbeit Macht Frei - Work sets you free.

Netanyahu stressed the historical importance the sign has to Jews, saying that Poland must find those who desecrated the place where more than 1 million people, mostly Jews, were killed during World War II.

Meanwhile, there are still no clues 48 hours after the theft.

While Poland is enlisting its finest investigators to solve the crime, its bizarreness is proving fertile ground for conspiracy theories on the Internet and in official responses.

Three of the usual suspects were immediately mentioned: the Jews, the Germans and the Russians. The infamous metal sign is so linked with the Jewish people, it has been said, it must be on its way to Jerusalem. Another theory: The Russians want to embarrass the Poles - or maybe the Poles wanted to embarrass the Russians before the 65th anniversary in January of the liberation of Auschwitz by the Red Army. The Germans are suspected of wanting to bring home something that belongs to them, in their opinion.

There are four main avenues of investigation: hooliganism or sale for scrap metal or an act of neo-Nazism or a service for an art collector. The theft may also be the work of nationalist elements upset by the plan to invest more funds in renovating the site.

The police are working on sealing Poland's borders to prevent the historic sign from being spirited out of the country. The sign is symbolic not only for Jews, but also for the Poles, as one of their most important symbols of World War II and the destruction of the Polish people.

The director of the Auschwitz-Birkenau memorial museum, visibly shaken, said he believes the theft was the work of professionals. "I think it was done by specialists," Piotr Cywinski said. "It was a very well-prepared action."

British historian Andrew Roberts said the sign would generate huge interest on the burgeoning market for Nazi memorabilia.

The phrase "Arbeit Macht Frei" appeared at the entrances of other Nazi camps, including Dachau and Sachsenhausen, but the long, curving sign at Auschwitz was the best known.

Friday, President Shimon Peres met with Poland's Prime Minister Donald Tusk in Copenhagen and asked him to take all necessary measures to capture the thieves and to restore the sign to its rightful place. "The sign has deep historical significance to the Jewish people and to the entire world. It serves as a memorial for more than a million Jews who were exterminated at Auschwitz," Peres said.

Tusk said his government is putting all its efforts in investigating the theft, which is being treated as top priority. "The sign's theft is a very serious act, and it is as painful for us as it is for you," Tusk said.

"I am shocked and outraged by the theft of a recognizable symbol of Nazi cynicism and cruelty," Polish President Lech Kaczynski said in a statement, who appealed "to all my compatriots" to help the authorities.

Information and Diaspora Minister Yuli Edelstein said that "this is oneof the Polish police's biggest failures," adding that" anti-Semitic events in the world are multiplying, and there is valid fear for the safety of the Jews in the Diaspora." Noah Flug, who heads an umbrella organization of Holocaust survivors' advocacy groups in Israel, said he hopes the Polish police direct major efforts to solving the crime and bringing the perpetrators to justice. The Simon Wiesenthal Center in Los Angeles urged Poland to intensify its investigation and bring the thieves to justice. Its founder, Marvin Hier, said the sign was "the defining symbol" of the Holocaust.

Foreign Ministry official Yossi Levy said Israel was "astounded and angry" about the theft at Auschwitz. He said: "Israel has full faith in the Polish authorities in charge of the investigation, and believes that the Polish police will apprehend the inhuman thieves and restore the sign to its place to serve as a chilling testimony of the horrors committed at the camp." Police are offering 5,000 zlotys ($1,700) for information about the theft; the private security firm Art-Security group is offering double that, and the museum itself offered a 100,000-zloty ($34,000-dollar) reward for information, a spokesman said late Friday.

"This [theft] is very saddening," said Jaroslaw Mensfelt, the museum's spokesman. "The thieves either didn't know where they were or what's even worse, they did know but that didn't prevent them from stealing."