Police said initial information indicated three people had been inside the van. They forced the two security guards at the building to leave before they reversed the van into the front entrance, smashing the door.
The two security guards were giving testimony to police.
The assailants then triggered an incendiary device inside the van that police said appeared to have consisted of camping gas canisters and several containers of gasoline.
The burned-out van still stood outside the blackened entrance later Monday morning, as police forensic teams examined it for evidence.
Authorities said no warning call had been made before the attack. There was no immediate claim of responsibility.
Several small extremist or domestic terrorist groups are active in Greece, usually targeting official buildings, banks or symbols of state power with small bombs or incendiary devices.
The attacks usually occur late at night and rarely cause injuries.