Visitors walk through the entrance gate of the Auschwitz concentration camp in Oswiecim, southern Poland, in 2005. (Herbert Knosowski/Associated Press)Police investigating the theft of the infamous entrance sign from the former Auschwitz death camp are looking into the possibility that it may have been commissioned.

The original iron sign bearing the Nazi slogan "Arbeit Macht Frei " (German for "Work Shall Set You Free") that spanned the main entrance to the former death camp was stolen Friday and located in three pieces in northern Poland on Sunday.

Five men were arrested on Sunday, but police said none of the suspects has known ties to neo-Nazi or far-right groups and they appeared to be common criminals.

The sign was stolen early Friday morning from the museum at the site of the former Auschwitz death camp, where more than one million people, mostly Jews, were killed or died of starvation and disease while carrying out forced labour at the camp, which the Nazis built in occupied Poland.

Dariusz Nowak, a spokesman for police in Krakow, said the sign's historical significance makes it likely someone other than the alleged thieves had a role in the theft.

"[It] indeed it looks like someone is behind it," said Nowak.

Police officers present parts of the retrieved inscription from the Auschwitz Birkenau entrance, during a news conference in Krakow, Poland. (Alik Keplicz/Associated Press)

Police probing foreign link

Artur Wrona, the chief prosecutor in Krakow, said that based on the evidence gathered so far, the crime appears to have been commissioned by a "person living outside Poland."

Polish media have reported — without citing any sources — that a person living in Sweden could be under suspicion. Nowak said foreign police have been notified and were working on the case but refused to elaborate.

A Swedish police official in Stockholm said they have not yet been contacted.

"There has been no requests made by the Polish police to the Swedish police yet," superintendent Bertil Olofsson of the Swedish National Criminal Police said. "And so we can't confirm this speculation."

3 men have confessed to crime: police

Three of the five men men who have confessed to the theft were taken back to Auschwitz to show investigators how they unscrewed and tore the five-metre-long sign, which weighs 30 kilograms, from the gateposts.

A replica of the sign — made when restoration work was being done on the original — was hung on Friday in place of the missing sign. Non-Jewish Polish inmates made the original sign in Auschwitz in 1940.

The slogan was also used at the entrances to other Nazi death camps during the Holocaust, including Dachau and Sachsenhausen.

Today the site serves as one of the main tourist draws in southern Poland, attracting more than one million visitors per year.

With files from The Associated Press