Olli Heinonen, a former deputy director of the agency, said in an interview that while “it is clear that a facility of sizable scale cannot simply be erased in three weeks’ time without leaving traces,” the more likely risk is that the Iranians would pursue smaller-scale but still important nuclear work, such as manufacturing uranium components for a nuclear weapon.

“A 24-day adjudicated timeline reduces detection probabilities exactly where the system is weakest: detecting undeclared facilities and materials,” he said.

David Albright, the president of the Institute for Science and International Security and a former weapons inspector in Iraq, also said that three weeks might be ample time for the Iranians to dispose of the evidence of prohibited nuclear work. Among the possibilities, he said, were experiments with high explosives that could be used to trigger a nuclear weapon, or the construction of a small plant to make centrifuges.

“If it is on a small scale, they may be able to clear it out in 24 days,” Mr. Albright said in a telephone interview. “They are practiced at cheating. You can’t count on them to make a mistake.”