The AP says there was no immediate claim of responsibility for the attack and that authorities received no warning.
According to the AP story, police said three people were inside the van. The attackers forced two security guards out of the entrance to the building, then reversed the van and smashed into the front door, and then triggered an "incendiary device" that apparently consisted of camping gas canisters and several containers of gasoline. The van had been stolen about 40 days earlier.
Microsoft has about 125 employees in Greece.