But Amos Yadlin, the military intelligence chief at the time, told reporters on Wednesday that Israel had only the vaguest idea what was happening until months before the attack. Comparing the effort to a jigsaw puzzle, Mr. Yadlin said, “if you basically say that 1,000 parts are giving you a good picture, in 2006, we had, like, 50 parts, only the first suspicions that there is a nuclear program in Syria.”
Mr. Yadlin, who now directs the Institute for National Security Studies in Tel Aviv, said that after a Mossad operation yielded something like “500 parts of the puzzle,” then “we immediately understood that we are facing a project of a plutonium nuclear reactor which had only one purpose: to produce a nuclear bomb.” From that point, he said, Israel assessed correctly that it had six to nine months to strike before the reactor became operational.
According to a vivid 2012 account in The New Yorker, the breakthrough came in March 2007, when Mossad agents raided the Vienna home of Ibrahim Othman, the head of the Syrian Atomic Energy Commission, and extracted valuable information from his computer. Although Israeli officials confirmed the raid, the details remain classified in Israel.
Israeli officials calculated that since so few people in Syria knew of the reactor, if Israel carried out an attack and did not take responsibility, Mr. Assad could deny it had happened and not strike back.
“Almost nobody in Syria knew about it, not even the chief of staff and the defense minister,” Mr. Yadlin said of the reactor.
Amir Peretz, Mr. Barak’s predecessor as defense minister, was involved in the early planning of the raid and related discussions with American officials. He said in an interview that the Israelis would have preferred for the United States to have destroyed the reactor, but quickly understood that they were on their own.
Western intelligence agencies had missed the signs of the project, partly because, from satellite pictures, it looked like a farm, he said.
“Usually, when somebody sets up a nuclear facility, it is surrounded by fences, antiaircraft batteries, soldiers,” Mr. Peretz said. “This was open — there were animals wandering around.”