It's already being called "an accident" by the Iranian press. But is it?

BBC:

Fifteen soldiers have been killed in a huge explosion at a military base near Iran's capital Tehran, officials say.

The blast occurred when weapons were being moved inside a Revolutionary Guards depot, a commander from the elite unit told state TV.

Windows in nearby buildings were shattered and the blast was heard in central Tehran, 40 km (25 miles) away.

Two hours after the explosion a fire still raged and there were traffic jams on nearby roads, a local reporter said.

Local MP Hossein Garousi said "a large part of an ammunition depot exploded," parliament's website reported.

Revolutionary Guards commander Ramezan Sharif did not say what had caused the "accident" in the village of Bidganeh, near the city of Karaj.

"Some of the casualties are reported to be in a critical condition," he said.

An emergency worker said that 12 people had been taken to hospital.

So why would we doubt the truthfullness of the accident explanation.

First, there are the internal enemies. Other factions opposed to Supreme Leader Khamenei may be trying to embarrass him. Or it could be domestic terrorists, although none of the groups active in Iran have demonstrated this kind of military capability. You just don't waltz in and blow up a munitions dump. Sophisticated planning is necessary to do a job like that and Iran's Arab, Kurdish, and other minority terrorist groups haven' gotten much past the car bomb stage.

That leaves foreigners; Israel and the US. It would help if we knew what was being stored in those buildings. If there were sophisticated surface to air missiles, that would point to Israel.

Note: This is the second Rev Guard munitions storage facility to blow up in the last two years.

There have been occasional unexplained explosions in Iran before.

Eighteen people were killed in a blast at a Revolutionary Guards base in the north-western Lorestan province in October 2010.

Either these guys are incredibly careless and stupid, or someone is trying to send a message. Neither scenario can be ruled out.