The Western diplomat said the Afghan man was initially given a sizable sum of money to take part in the talks  and to help persuade him to return.

While the Afghan official said he still harbored hopes that the man would return for another round of talks, American and other Western officials said they had concluded that the man in question was not Mr. Mansour. Just how the Americans reached such a definitive conclusion  whether, for instance, they were able to positively establish his identity through fingerprints or some other means  is unknown.

As recently as last month, American and Afghan officials held high hopes for the talks. Senior American officials, including Gen. David H. Petraeus, said the talks indicated that Taliban leaders, whose rank-and-file fighters are under extraordinary pressure from the American-led offensive, were at least willing to discuss an end to the war.

The American officials said they and officials of other NATO governments were helping to facilitate the discussions, by providing air transport and securing roadways for Taliban leaders coming from Pakistan.

Last month, White House officials asked The New York Times to withhold Mr. Mansour’s name from an article about the peace talks, expressing concern that the talks would be jeopardized  and Mr. Mansour’s life put at risk  if his involvement were publicized. The Times agreed to withhold Mr. Mansour’s name, along with the names of two other Taliban leaders said to be involved in the discussions. The status of the other two Taliban leaders said to be involved is not clear.

Since the last round of discussions, which took place within the past few weeks, Afghan and American officials have been puzzling over who the man was. Some officials say the man may simply have been a freelance fraud, posing as a Taliban leader in order to enrich himself.

Others say the man may have been a Taliban agent. “The Taliban are cleverer than the Americans and our own intelligence service,” said a senior Afghan official who is familiar with the case. “They are playing games.”