The government of Iran released five senior members of Al Qaeda earlier this year, including the man who stepped in to serve as the terrorist group’s interim leader immediately after Osama bin Laden’s death, and who is the subject of a $5 million bounty, according to an American official who had been briefed on the matter. Iran’s release of the five men was part of a prisoner swap in March with Al Qaeda’s branch in Yemen, the group holding an Iranian diplomat, Nour Ahmad Nikbakht. Mr. Nikbakht was kidnapped in the Yemeni capital of Sana in July 2013.