An explosion that rattled Iran's capital came from an area in its eastern mountains that analysts believe hides an underground tunnel system and missile production sites, satellite photographs show.
What exploded in the incident early on Friday that sent a massive fireball into the sky near Tehran remains unclear, as does the cause of the blast.
The unusual response of the Iranian government in the aftermath of the explosion, however, underscores the sensitive nature of an area near where international inspectors believe the Islamic Republic conducted high-explosive tests two decades ago for nuclear weapon triggers.
The blast shook homes, rattled windows and lit up the horizon early on Friday in the Alborz Mountains. State TV later aired a segment from what it described as the site of the blast.
One of its journalists stood in front of what appeared to be large, blackened gas cylinders, though the camera remained tightly focused and did not show anything else around the site. Defence Ministry spokesman Davood Abdi blamed the blast on a leaking gas he did not identify and said no one was killed in the explosion.
Abdi described the site as a "public area," raising the question of why military officials and not civilian firefighters would be in charge. The state TV report did not answer that.
Satellite photos of the area, some 20km east of Tehran, showed hundreds of metres of charred scrubland not seen in images of the area taken in the weeks ahead of the incident. The building near the char marks resembled the facility seen in the state TV footage.
The gas storage area sits near what analysts describe as Iran's Khojir missile facility. The explosion appears to have struck a facility for the Shahid Bakeri Industrial Group, which makes solid-propellant rockets, said Fabian Hinz, a researcher at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies in Monterey, California.
The Washington-based Center for Strategic and International Studies identified Khojir as the "site of numerous tunnels, some suspected of use for arms assembly." Large industrial buildings at the site visible from satellite photographs also suggest missile assembly being conducted there.
The US Defense Intelligence Agency says Iran overall has the largest underground facility program in the Middle East.