Polish police and border guards stepped up security checks at airports and border crossings Saturday as the search intensified for the infamous sign stolen from the Auschwitz death camp memorial.

The brazen overnight theft of one of the Holocaust's most chilling and notorious symbols early Friday sparked outrage from around the world, and Polish leaders declared recovering the 16-foot sign a top priority. The sign read "Arbeit Macht Frei" — work makes you free — a grim Nazi slogan etched in the minds of millions.

Interior Minister Jerzy Miller ordered police to question all possible witnesses and suspects in a nationwide effort to find the sign.

The director of the Auschwitz-Birkenau memorial museum, who was visibly shaken by the dramatic theft, told The Associated Press he believes it was carried out by professionals and that none of the memorial museum's staff are considered suspects.

"I think it was done by specialists," Piotr Cywinski said. "It was a very well-prepared action."

Security guards patrol the site around the clock, but due to its vast size they only pass by any one site at intervals. He said that gave thieves between 20 to 30 minutes to remove the sign and carry it off.

Museum authorities said they don't know how much the sign weighs exactly but spokesman Pawel Sawicki said it is made of hollow steel pipes and is believed to weigh only around 65 to 90 pounds.

"A single person could lift it," Sawicki said.

Sawicki said the entire Auschwitz staff was deeply shaken by the theft. He defended security at the camp but said no one could have ever imagined thieves seizing on the gate's sign.

"Thieves are also able to robs banks and museums. Clearly this was well planned. It's a blow to our human heritage," Sawicki said.

The Simon Wiesenthal Center, a leading Jewish human rights group, urged Poland to intensify its investigation and bring the thieves to justice.

"The fact is that the 'Arbeit Macht Frei' sign has become the defining symbol of the Holocaust, because everyone knew that this was not a place where work makes you free, but it was the place where millions of men, women, and children were brought for one purpose only — to be murdered," said Rabbi Marvin Hier, the center's founder and dean.

More than 1 million people were killed in gas chambers, from forced labor, or starvation and disease at the camp set up by the Nazis in occupied Poland during World War II. Most of the victims were Jews but they also included Gypsies, Poles, homosexuals and political prisoners.

The camp was liberated by the Soviet army on January 27, 1945. Polish officials plan to mark the 65th anniversary of that liberation next month with somber ceremonies at the site.