They don't know much as of yet other than it looks like no one was hurt and not to leap to any conclusions. A newspaper received a bomb warning 30 minutes before the blast, plus today was to see the start of a seties of transport strikes. Doesn't sound like the usual lot.
Add to that the fact it was at 6am local time (hardly a casualty maximisation strategy), that there was another attack on a ministerial building just six months ago, and that members of the Greek 17 November terrorist/revolutionary group have just begun a fresh trial and this looks like it could well be domestic terrorism - in fact, that seems by far the most likely.
Anyway, some Greek blogs in English that may be able to provide a bit more info:
This is not my country
Sovereignty of SeaWitch