But the Iranians have reason to suspect they are high on the target list: in the past, they have found evidence of sabotage of imported equipment, notably power supplies to run the centrifuges that are used to enrich uranium at Natanz. The New York Times reported in 2009 that President George W. Bush had authorized new efforts, including some that were experimental, to undermine electrical systems, computer systems and other networks that serve Iran’s nuclear program, according to current and former American officials.

The program is among the most secret in the United States government, and it has been accelerated since President Obama took office, according to some American officials. Iran’s enrichment program has run into considerable technical difficulties in the past year, but it is not clear whether that is because of the effects of sanctions against the country, poor design for its centrifuges, which it obtained from Pakistan, or sabotage.

“It is easy to look at what we know about Stuxnet and jump to the conclusion that it is of American origin and Iran is the target, but there is no proof of that,” said James Lewis, a senior fellow at the Center for Strategic and International Studies in Washington and one of the country’s leading experts on cyberwar intelligence. “We may not know the real answer for some time.”

Based on what he knows of Stuxnet, Mr. Lewis said, the United States is “one of four or five places that could have done it  the Israelis, the British and the Americans are the prime suspects, then the French and Germans, and you can’t rule out the Russians and the Chinese.”

President Obama has talked extensively about developing better cyberdefenses for the United States, to protect banks, power plants, telecommunications systems and other critical infrastructure. He has said almost nothing about the other side of the cybereffort, billions of dollars spent on offensive capability, much of it based inside the National Security Agency.

The fact that the worm is aimed at Siemens equipment is telling: the company’s control systems are used around the world, but have been spotted in many Iranian facilities, say officials and experts who have toured them. Those include the new Bushehr nuclear power plant, built with Russian help.

But Bushehr is considered by nuclear weapons experts to be virtually no help to Iran in its suspected weapons program; there is more concern about the low-enriched uranium produced at Natanz, which could, with a year or more of additional processing, be converted to bomb fuel.