Iranian media says it was caused by a fire at an “explosives producing factory” east of Tehran. Iranian dissidents say that the “fire” was suspiciously close to the Parchin military base, a site that the UN’s nuclear inspectors have been interested in for nearly 10 years. (Iran’s been accused of using the site to test technology needed for, er, atomic explosions.) And it wasn’t just a fire that broke out; if you believe the dissidents, there was an explosion so gigantic that it broke windows nine miles away. And produced a “glare” visible from great distances.

Wouldn’t be the first time in the past few years either than an Iranian military site has suffered an enormous mysterious explosion resulting in multiple fatalities. More from the Times of Israel:

Several arms facilities and military bases are located east of the Iranian capital, including Parchin. UN nuclear inspectors have been seeking to visit the site to answer concerns about Iran’s atomic program.

The base lies at the centre of allegations of past Iranian research into sophisticated explosives that can be used to detonate a nuclear warhead.

Tehran, which has denied inspectors access to Parchin since 2005, insists its nuclear program is for purely civilian uses. Israel and the West fear Iran is seeking to attain nuclear weapons.

In August Iran reiterated that it will not allow IAEA inspectors to visit the site.

Yep — the same site that Iran’s been shielding from suspicious UN investigators for nine long years allegedly goes up like a roman candle, just a few weeks after Iran once again denied IAEA requests to inspect it and with the west possibly on the brink of a sell-out nuclear deal with Tehran that would place the site off-limits. Hmm!

Three possibilities here. One: The dissidents are lying. Someone looking to stir things up saw the Iranian media report about a fire, realized that Parchin is east of Tehran, and seized the opportunity to spin a damning tall tale about a mammoth boom originating from the same site that has the UN worked up. Now the IAEA will be even more determined to investigate Parchin, which means more pressure on the regime.

Two: It happened but it was a bona fide accident. If Parchin really is a test site for atomic weapon technology, go figure that a test might occasionally go bad. Which raises the question of just what sort of explosion this was. If it broke windows miles away and emitted a bright glare, that could mean either a really large conventional blast (e.g., if the fire reached the base’s weapons depot) or a small atomic blast — and of course western governments who detected it would have an interest in hushing it up too, lest they’re forced to admit that they failed to stop Iran from getting the bomb. But if there really was a huge explosion, how come social media wasn’t instantly inundated with “whoa!” tweets from Iranians living in and around east Tehran and Parchin? Seems hard to believe Iran’s Internet censorship could be so thorough that no trace of a reaction like that was detected online by western media. Which means maybe there was no such reaction, and thus no explosion.

Three: Israel did it! That crafty son of a gun Bibi acted below the radar to sabotage Parchin, either through Mossad infiltration (a la Stuxnet) or through an air attack that neither side wants to acknowledge. But if that’s how it happened, why doesn’t Iran just accuse Israel publicly of it? I know, I know — authoritarian regimes don’t like to acknowledge that they’re vulnerable militarily, especially as regards the dreaded Zionist entity. Once they admit they’re weak vis-a-vis Israel, Iran’s domestic dissidents might calculate that they’d be weak in the face of a new uprising a la the Green Revolution too and suddenly the mullahs have their hands full again with protests. Understood, but by accusing Israel of a secret attack at a moment when the White House is desperate to make a deal with Tehran on nukes, Iran would drive the wedge between Obama and Netanyahu even deeper. According to Eli Lake’s sources, Iran has gone so far lately as to tell its Quds Force operatives in Iraq not to make trouble for the U.S. military there; that’s supposed to be a “confidence-building gesture” towards a deal with the U.S. that would see sanctions against Iran lifted. Iran could make that deal even sweeter for itself by trying to isolate Israel as part of it, blaming Netanyahu for essentially trying to sabotage America’s efforts towards peace. And if they’re worried about perceptions of vulnerability, they could simply say that there was minimal damage from the attack, that a bunch of Zionist infiltrators were captured and executed, and that Parchin is operating normally. Not sure why they don’t do that — unless this whole “giant explosion” story is made up by regime critics whole cloth.