WASHINGTON — Osama bin Laden, the mastermind behind the Sept. 11 attacks against the United States, is dead, and the U.S. is in possession of his body, a person familiar with the situation said late Sunday.

President Barack Obama was expected to address the nation on the developments Sunday night.

It was unclear where how bin Laden was killed and how the U.S. captured his body. Officials have long believed bin Laden, the most wanted man in the world, was hiding a mountainous region along the Pakistan-Afghanistan border.

The person spoke on the condition of anonymity in order to speak ahead of the president.