When the United States and Iran swapped prisoners last week, nothing was said to resolve the mystery about another captive: Robert A. Levinson, a Central Intelligence Agency consultant who disappeared in Iran in 2007.

Iranian leaders have long said that they knew nothing about the missing American, and United States officials have said that he may no longer be in Iran — or even still alive. Aside from a hostage video and photographs of him in an orange jumpsuit five years ago, there had been no public clues about his fate.

But newly disclosed documents suggest that Iranian officials knew far more about Mr. Levinson. In late 2011, a top Iranian diplomat acknowledged that his country was holding the American and would release him if the United States helped delay an assessment criticizing Iran’s nuclear activities, the documents say.

Iran’s ambassador to France at the time, Seyed Mehdi Miraboutalebi, made the statement during a private gathering at his Paris residence with two men working with an American religious organization, according to a report about the session. The meeting resulted from a letter sent to Ayatollah Ali Khamenei, Iran’s supreme leader, by a leader of the religious group, the Fellowship Foundation, which had previously helped win the release of an Iranian-American imprisoned in Tehran.