Greek police are examining an envelope found at a central Athens spot indicated by an anonymous caller, claiming responsibility for a bomb attack by militants last month.

Police said Monday the envelope contained a memory stick which is being examined by forensic experts. There was no information on its contents.

The time bomb exploded Nov. 24 outside the Federation of Greek Enterprises building, near parliament in central Athens, causing damage but no injuries.

The nighttime blast smashed windows in several nearby buildings, including a hotel, private apartments, and the embassy of Cyprus. Police had cordoned off the area following warning telephone calls.

Militant anarchist and far-left groups have carried out bomb attacks in Greece for decades, frequently targeting banks and the offices of large companies.