ATHENS -- Attackers targeted the Microsoft main office in the Greek capital in a pre-dawn arson attack Wednesday, smashing the entrance with a mini-van and blowing up gas canisters, police said. Anti-terrorist police were investigating, and authorities did not rule out an extremist group was behind the attack, which caused no casualties. Two or three masked men immobilized two security guards around 3:45 a.m. local time, then rammed the stolen mini-van through the entrance, and set fire to the van, which was packed with gas canisters, police said. The fire and explosion damaged the entrance and ground floor of the building, the Greece corporate headquarters of the U.S. technology giant, located in the northern Athens suburb of Maroussi. "We are grateful that there were no injuries and our staff are safe," the company said in a statement, noting that the assailants had forced the security guards out of the building before setting the mini-van on fire. "We await the completion of the police investigation to form a more complete picture on the case," they added. A fire department spokeswoman told AFP: "Serious damage has been caused to the ground floor of the building. Our first estimate is in the region of 60,000 euros ($75,000)." The mini-van used in the attack was entirely gutted, she added. The offices will remain closed on Wednesday and Thursday, a company source said. Attacks against targets such as diplomatic missions, state offices, companies and banks, which usually do not cause casualties, are frequent in Greece and blamed by police on extremist and anarchist groups. In April, a previously unknown group called "Revolutionary Action" claimed responsibility for an attack with an incendiary device that badly damaged an annex of the Greek ministry of administration in Athens.