Olli Heinonen, a former deputy director general of the IAEA, said Iran may lack the precision engineering capabilities needed to build a working bomb. In theory, however, Iran could secretly construct a bomb or warhead, a process likely to take several years, and then produce weapons-grade fuel in a month or so.

“You can build everything else, but you just leave the nuclear material out and do that at the end,” said Heinonen, now a senior fellow at the Belfer Center for Science and International Affairs at Harvard University.

Others warn that spy satellites, surveillance drones and other sources may not be sufficient to spot a secret bomb program given Iran’s sprawling geography — one fifth the size of the continental U.S. — and large industrial base.

“Iran is a big country, and the idea that you can have blanket coverage — you can’t,” said Mark Lowenthal, a former senior CIA and State Department intelligence analyst.

Still, U.S. spy services and their allies have a substantial espionage effort aimed at Iran.

Analysts study business and procurement orders by Iranian companies, eavesdrop on government communications and monitor large construction and mining projects. Airborne sensors help analyze electromagnetic, chemical and other emissions that can offer telltale clues of what’s happening inside factories.

On the other hand, U.S. intelligence agencies were unaware that Syria was secretly constructing a nuclear reactor until shortly before Israeli warplanes destroyed the partially built facility in 2007, Rep. Rogers said.

The Iranians “are learning from their mistakes, and they are getting better about how to keep things more quiet,” he said. “This is a cat-and-mouse game for them.”

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