In addition to confirming the deaths for the first time, the Islamic State announced in an audio message, released on the Telegram app, that Mr. al-Baghdadi had been succeeded as leader by Abu Ibrahim al-Hashemi al-Qurayshi, whom it identified as the “emir of the believers” and “caliph.”

But almost nothing is publicly known about Mr. al-Qurayshi, including his real name, and counterterrorism analysts were scrambling Thursday to try to figure out who he is.

Quotable: “Nobody — and I mean nobody outside a likely very small circle within ISIS — have any idea who their new leader ‘Abu Ibrahim al-Hashimi al-Qurayshi’ is,” Paul Cruickshank, editor of the CTC Sentinel at the Combating Terrorism Center, said.

Possibilities: One analyst told us ISIS leaders often acquire a new nom de guerre with the appointment to a new position, meaning Mr. al-Qurayshi may have had a completely different name last week.