A preliminary investigation into British historian David Irving's alleged wrongdoings under Austrian law that makes Holocaust denial a crime was concluded on Friday, a judge said. Irving, an expert on the Third Reich who has claimed that Adolf Hitler knew nothing about the systematic slaughter of 6 million Jews, was detained November 11 in the southern province of Styria on a warrant issued in 1989. The preliminary investigation report, which has not been made public, will now be handed to prosecutors who will decide whether to file charges, said Christian Gneist, a judge at the Vienna Regional Court. Irving, 67, remains in custody in Vienna pending the prosecutors' decision, Gneist said. Austrian law allows for suspects to be detained initially for 14 days, but the time can be extended after legal reviews, Gneist said. If convicted, Irving could face up to 20 years in prison. In a statement posted on his Web site, Irving's supporters said he was arrested while on a one-day visit to Vienna, where they said he had been invited "by courageous students to address an ancient university association."