Poland's history institute says that newly seized documents suggest former president and Solidarity hero Lech Walesa was an informer.
The documents were taken earlier this week from the home of a former communist-era interior minister, Gen Czeslaw Kiszczak.
Lukasz Kaminski, head of the Institute of National Remembrance, said the documents appear authentic.
Mr Walesa has long denied being an informer in the 1970s.
The former president said the new materials could not originate from him, according to Polish radio.
The 279 pages of documents have not yet been properly analysed, and will be made public in due course, Mr Kaminski said.