The little-known Terrorist Guerrilla Group and the Conspiracy of the Cells of Fire, of which five suspected members were arrested earlier this year, yesterday posted a message on the Internet claiming responsibility for a blast at the headquarters of the Ethniki Insurance firm in Athens. The blast occurred at about 11 p.m. on Sunday night and was heard in many parts of central Athens. Officers said that a device containing some 3 kilos of dynamite was placed outside the building, which is located in Neos Cosmos toward the northern end of busy Syngrou Avenue, not far from the center of Athens. Nobody was hurt in the explosion but it destroyed the ground floor of the building, where a branch of the National Bank of Greece was also located, and smashed the windows of several cars parked close by. Some adjacent buildings also suffered minor damage. Police had sealed off the area after warning was given of the imminent explosion in an anonymous telephone call to the Eleftherotypia daily newspaper 15 minutes before the bomb went off. Initially it was thought that Revolutionary Struggle had planted the bomb as it has carried out similar attacks in the past. However, the joint claim of responsibility by the Terrorist Guerrilla Group and the Conspiracy of the Cells of Fire seemed to douse this theory. The two groups claimed an attack on the home of PASOK MP Mimis Androulakis last month. In the online statement, the groups gave details about how the attack was carried out. Fragments of the explosive device were recovered yesterday by forensic experts and officers will also be examining CCTV footage.