There's been a mysterious and possibly deadly explosion at an Iranian facility that the US and international monitors believe was once used to test nuclear weapons components — and that Tehran has barred International Atomic Energy Agency inspectors from visiting.

According to The New York Times, an explosion at the Parchin military installation caused an "enormous orange flash that illuminated Tehran." Iranian officials "confirmed that two people were missing after 'an ordinary fire' caused by 'chemical reactions of flammable material,'" according to the Times account.

But this is hardly the only suspicious explosion to hit a sensitive Iranian military facility, and it's unlikely that Iran would admit to an act of sabotage. In 2011, the architect of Iran's ballistic missile program was killed in a suspicious blast. And there have been several assassinations of Iranian nuclear scientists in recent years, killings that have been blamed on both Israel and the Mujahideen el-Khalk, an anti-regime militant group.

And Parchin was apparently home to infrastructure needed to develop ballistic triggers for a uranium-based nuclear detonation, work that apparently took place at the facility prior to 2004 when these activities were discovered and made public by the US and international regulators.

Iran is currently barring international inspectors from visiting Parchin. It's a place of potential significance to Iran's nuclear program; if today's explosion was in fact an assassination it would suggest that some kind of sensitive work is still going on there. The question is what that work could be — and what Parchin's significance might be to a nuclear program whose final status is still being negotiated by Iran, the US, and its international partners.

According to David Albright, a physicist and the founder and president of the Institute for Science and International Security, Parchin has some of the most sophisticated ballistics technology in Iran, including slow-motion diagnostic cameras needed for the close analysis of projectiles and explosives. Parchin is the only place in Iran known to have these research and development capabilities — which means that if Iran were still trying to develop a trigger for a future nuclear device, it would be doing it either at Parchin or at some other, as-et undiscovered complex.

"It's the logical place for it to occur because you need special facilities to handle high explosives, bunkers to store them, diagnostic equipment to analyze the experiments ... and it's not easy to create that infrastructure," Albright told Business Insider. Parchin might even have a high-explosives chamber where Iranian researchers may once have been planning to test a mock-up of an atomic bomb, Albright explained.

But proof of any of these activities ceased years ago, according to Albright. The ban on IAEA inspectors aside, he thinks it's unlikely that Iran is continuing with the kind of research and development it was performing at Parchin before the US and others went public with their suspicions of the facility's purpose in 2004.

That doesn't mean the site isn't of interest to potential saboteurs or international inspectors, though. "The people there may have had some significance. People who worked in these alleged activities may still be there. and some of the buildings are still there."

Understanding Iran's capabilities at Parchin is impossible without the site being opened to inspectors, Albright says. "You need an Iranian decision to cooperate to really understand these activities," he says. "They're very small-scale and really hard to detect."

If yesterday's explosion was sabotage, then at least one player in the Iranian nuclear drama is still deeply suspicious of whatever's still going on there.