The head of Poland's historical institute says its prosecutors are investigating the veracity of documents the institute has released that purport to show that Solidarity founder Lech Walesa collaborated with the communist regime in the 1970s for money.

Walesa, 72, denies he informed the security police on anyone or took money from the government, and says the documents are forged.

Lukasz Kaminski, head of the National Remembrance Institute, said Thursday that based on Walesa's statement, special prosecutors have opened a probe into documents alleging that Walesa collaborated under the codename "Bolek." Among them are receipts for money signed "Bolek."

Last week the institute seized the document from the home of the last communist-era interior minister, acting on a law that requires key historical document to be handed in.