WASHINGTON — The huge explosion that destroyed a major missile-testing site near Tehran three weeks ago was a major setback for Iran’s most advanced long-range missile program, according to American and Israeli intelligence officials and missile technology experts.

In interviews, current and former officials said surveillance photos showed that the Iranian base was a central testing center for advanced solid-fuel missiles, an assessment backed by outside experts who have examined satellite photos showing that the base was almost completely leveled in the blast. Such missiles can be launched almost instantly, making them useful to Iran as a potential deterrent against pre-emptive attacks by Israel or the United States, and they are also better suited than older liquid-fuel designs for carrying warheads long distances.

It is still unclear what caused the explosion, with American officials saying they believe it was probably an accident, perhaps because of Iran’s inexperience with a volatile, dangerous technology. Iran declared it an accident, but subsequent discussions of the episode in the Iranian news media have referred to the chief of Iran’s missile program as one of the “martyrs” killed in the huge explosion. Some Iranian officials have talked of sabotage, but it is unclear whether that is based on evidence or surmise after several years in which Iranian nuclear scientists have been assassinated on Tehran’s streets, and a highly sophisticated computer worm has attacked its main uranium production facility.

Both American and Israeli officials, in discussing the explosion in recent days, showed little curiosity about its cause. “Anything that buys us time and delays the day when the Iranians might be able to mount a nuclear weapon on an accurate missile is a small victory,” one Western intelligence official who has been deeply involved in countering the Iranian nuclear program said this weekend. “At this point, we’ll take whatever we can get, however it happens.”