He was told that the question, posed through an intermediary, had come from Osama bin Laden: Will you be my bodyguard?

The man being asked was Ahmed Khalfan Ghailani, a Tanzanian who had been training in Qaeda camps in Afghanistan, and who had come to the attention of the Qaeda leader, according to a newly disclosed F.B.I. document.

Mr. Ghailani did not immediately say yes, the document shows. He said he would have to check with “the brothers at the training camp.” When he had not yet responded two weeks later, he was asked again.

This time, Mr. Ghailani said that he “did not have a good reason not to join them, and thus he became a bodyguard” for Mr. bin Laden, according to the Federal Bureau of Investigation’s account.