Poland's prime minister said Saturday that he had ordered an investigation into whether the CIA ran secret prisons for terrorism suspects in the country -- an allegation the government repeatedly has denied.

Kazimierz Marcinkiewicz said a detailed probe would be conducted to "check if there is any proof that such an event took place in our country. It is necessary to finally close the issue because it could be dangerous to Poland." Marcinkiewicz's spokesman, Konrad Ciesiolkiewicz, said he did not know who would carry out the investigation.

More than a half-dozen investigations are under way into whether European countries hosted secret U.S.-run prisons in which terrorism suspects were allegedly held, and whether European airports and airspace were used for CIA flights taking prisoners to countries where torture is practiced.

European Union leaders say any member states found to have been involved in such prisons could have their voting rights suspended. Secret facilities and harsh interrogation methods would violate European human rights conventions.

The Polish newspaper Gazeta Wyborcza reported Saturday that Gulfstream airplanes belonging to either the CIA or FBI landed at least five times at the Szczytno-Szymany airport in northeastern Poland since December 2002.

Only border control officials and minivans from Kiejkuty, the site of a training school for Poland's intelligence services, were allowed to approach the aircraft, a former airport employee told the newspaper.