Högström has denied plotting to steal the sign — whose German inscription translates as “Work Will Set You Free” — Boguslawa Marcinkowska, spokeswoman for prosecutors in Krakow, southern Poland, said without giving further details.
“The interrogation will continue until late Monday night and probably also on Tuesday,” Marcinkowska added, according to the Polish PAP news agency.
Högström, 34, was arrested in Stockholm on February 11 and has been charged in Poland with “plotting the theft” of the sign that has come to represent the horrors of Nazi Germany.
Polish police recovered the five-metre (16-foot) metal sign on December 20, two days after it was stolen. They arrested and charged five Polish men.
On March 11, a Stockholm court approved Högström’s extradition to Poland to face trial.
In 1994, Högström founded the National Socialist Front, a Swedish neo-Nazi movement he headed for five years before quitting.
He told Swedish media he was to act as an intermediary to pick up the sign and sell it to a buyer, adding however that he informed Polish police about the people behind the plot.
The sign, which had been cut into three parts, was returned by investigators to the Auschwitz museum on January 21, less than a week before commemorations for the 65th anniversary of the camp’s liberation by Soviet troops.
The sign has long symbolised the horror of the camp where some 1.1 million people — one million of them Jews — were victims of Nazi German genocide from 1940 to 1945.
Polish judicial authorities indicted Högström in January and issued an arrest warrant for him on February 2.