WASHINGTON — A spectacular explosion on Sunday night outside Tehran took place deep inside the Parchin military base, where Iran produces crucial elements of its missiles and other munitions, raising new questions about whether the blast was an accident or sabotage.
The explosion and resulting fire, which Iranian news organizations have described in only the most general terms, could be seen from apartments in Tehran and appeared to have destroyed several buildings. But it was distant from a part of the base to which the International Atomic Energy Agency has been seeking access for years, to investigate reports of experiments on high explosives that could have been used in nuclear weapons.
The agency’s evidence about that activity dates back more than a decade, and that part of the base has been so bulldozed and reconfigured in recent years that inspectors concede it is doubtful there is much left to see or test if they ever get access.
The explosion, according to satellite photographs from Airbus that were analyzed by the Institute for Science and International Security, took place in a densely built region toward the southern end of the base, in an area that appeared from past photographs to be littered with bunkers. The damage was reminiscent of pictures of a missile-development site 30 miles west of Tehran that was virtually destroyed during a test in November 2011 that killed 17 people, including Gen. Hassan Tehrani Moghaddam, the leading force behind Iran’s advanced missile efforts.
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At that time, Israel was widely suspected of sabotaging the base. Yet there was never definitive evidence, and no group has ever taken responsibility. (A senior Israeli officer noted several weeks later that the timing of the explosion was remarkable because General Moghaddam, who traveled often, “just happened to be sitting in his office” at the time.)
By comparison, the damage at Parchin on Sunday night was not as extensive. Nor were the reported casualties, perhaps because the explosion happened in the evening, when presumably there were fewer workers in the area.
Iranian news media reports, while sketchy, indicated that two people were missing. The Institute for Science and International Security, in its study, concluded that “at least six buildings appear damaged or destroyed.” But vast parts of the base appeared untouched.
It is possible that something other than sabotage caused the explosion, experts said Thursday. “It could have been an accident,” David Albright, the founder of the institute, said in an interview. American efforts to develop its own missile fleets decades ago were littered with accidents.
Even if the blast was an accident, the Iranians will almost certainly suspect foul play. The history of sabotage of Iranian nuclear and missile facilities, and assassinations of its leading scientists, is a long one. The United States played a central role in shipping faulty parts into Iran’s nuclear centrifuge facilities, and together the United States and Israel carried out a highly classified program, code-named Olympic Games, that used one of the most sophisticated cyberattacks in modern history to cause more than a thousand Iranian centrifuges to blow up four years ago, according to participants in that program. Neither the United States nor Israel has ever acknowledged a role in that attack.
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This summer, Iran’s foreign minister, Mohammad Javad Zarif, accused the West of trying to sabotage a heavy-water nuclear reactor under construction near the town of Arak by altering components of its cooling system, a step he said could have led to an “environmental catastrophe.”
“You know about cyberattacks,” Mr. Zarif said, noting that they had been aimed at the uranium production facilities at Natanz. Arak is a facility that, if it ever goes into operation, could produce plutonium, and it is of particular concern to the United States and Europe because it could provide another pathway to a bomb.
Mr. Zarif said a foreign power had tried “to create malfunctions in equipment” purchased for the Arak plant from outside Iran “so that, for instance, instead of cooling the facility, they would have increased the heat in the facility had we not detected it” in time.
But in interviews, when promised anonymity, American officials have insisted that Washington was not involved in the assassination of scientists, or in the 2011 missile explosion. And it has asked few questions, publicly or privately, about the causes of the 2011 explosion, other than noting that the materials being used at that plant were highly volatile.
The latest explosion, whether accidental or not, comes at a key moment in Iran’s confrontations with the West. It has resisted a series of efforts by the International Atomic Energy Agency to get answers on about a dozen separate issues, of which the activities at Parchin a decade ago were just one, and among the less sensitive. A trip to Tehran earlier this week, just after the explosion, by I.A.E.A. inspectors failed to make progress on that list, though the Iranians said the discussions were “very constructive.” Iran missed a deadline of Aug. 25 to provide a series of answers to the agency.
Working out those answers will be a part, American and European officials say, of any broader agreement with Iran to limit its nuclear activities. The deadline for that agreement is Nov. 24, and while Secretary of State John Kerry is scheduled to talk with his Iranian counterpart, Mr. Zarif, in Vienna on Wednesday, there is increasing concern among American negotiators that time is running short to negotiate what would be an enormously complex agreement. Among the hard-liners opposing a broader deal are the Revolutionary Guards, which oversee the activities at Parchin.