“They are certainly moving on that path, but we don’t believe they have actually made the decision to go ahead with a nuclear weapon,” Mr. Clapper told the Senate Select Committee on Intelligence.

Critics of the American assessment in Jerusalem and some European capitals point out that Iran has made great strides in the most difficult step toward building a nuclear weapon, enriching uranium. That has also been the conclusion of a series of reports by the International Atomic Energy Agency’s inspectors, who on Friday presented new evidence that the Iranians have begun enriching uranium in an underground facility.

Once Iran takes further steps to actually enrich weapons grade fuel — a feat that the United States does not believe Iran has yet accomplished — the critics believe that it would be relatively easy for Iran to engineer a warhead and then have a bomb in short order. They also criticize the C.I.A. for being overly cautious in its assessments of Iran, suggesting that it is perhaps overcompensating for its faulty intelligence assessments in 2002 about Iraq’s purported weapons programs, which turned out not to exist. In addition, Israeli officials have challenged the very premise of the 2007 intelligence assessment, saying they do not believe that Iran ever fully halted its work on a weapons program.

Yet some intelligence officials and outside analysts believe there is another possible explanation for Iran’s enrichment activity, besides a headlong race to build a bomb as quickly as possible. They say that Iran could be seeking to enhance its influence in the region by creating what some analysts call “strategic ambiguity.” Rather than building a bomb now, Iran may want to increase its power by sowing doubt among other nations about its nuclear ambitions. Some point to the examples of Pakistan and India, both of which had clandestine nuclear weapons programs for decades before they actually decided to build bombs and test their weapons in 1998.

“I think the Iranians want the capability, but not a stockpile,” said Kenneth C. Brill, a former United States ambassador to the International Atomic Energy Agency who also served as director of the intelligence community’s National Counterproliferation Center from 2005 until 2009. Added a former intelligence official: “The Indians were a screwdriver turn away from having a bomb for many years. The Iranians are not that close.”