THEY SLIPPED THROUGH the passage from the Scala Regia into the Chapel of the Blessed Sacrament, then hurried diagonally across the vast nave. Beneath the Monument to Alexander VII was a doorway leading into the Piazza Santa Marta. As they stepped outside into the bright sunlight, a roar of wild applause rose from St. Peter’s Square. The Pope had arrived for the General Audience. Angelli led Gabriel across the small piazza and into a gloomy-looking Baroque office building. In the lobby a nun sat motionless behind a reception table. She gave Gabriel and Angelli a disapproving look as they burst inside.

“Ibrahim el-Banna,” said Luca Angelli without elaboration.

The nun blinked twice rapidly. “Room four-twelve.”

They mounted the stairs, Angelli leading the way, Gabriel at his heels. When another swell of applause rose from the square, Gabriel gave Angelli a jab in the kidneys, and the Vatican security man began taking the steps two at a time. When they arrived at Room 412 they found the door was closed. Gabriel reached for the latch, but Angelli stayed his hand and knocked firmly but politely.

“Professor el-Banna? Professor el-Banna? Are you there?”

Greeted by silence, Gabriel pushed Angelli aside and examined the ancient lock. With the slender metal pick in his wallet he could have coaxed it open in a matter of seconds, but another roar of approval from the square reminded him there wasn’t time. He seized hold of the latch with both hands and drove his shoulder into the door. It held fast. He threw his body against the door a second time, then a third. On the fourth attempt, Angelli joined him. The wood of the doorjamb splintered, and they tumbled inside.

The room was empty. Not just empty, thought Gabriel. Abandoned. There were no books or files, no pens or loose papers. Just a single lettersized envelope, positioned in the precise center of the desk. Angelli reached for the light switch, but Gabriel shouted at him not to touch it, then pushed the Italian back into the corridor. He drew a pen from his coat pocket and used it as a probe to examine the density of the envelope’s contents. When he was reasonably certain it contained nothing but paper, he picked it up and carefully lifted the flap. Inside was a single sheet, folded in thirds. Handwritten, Arabic script:

We declare war on you, the Crusaders, with the destruction of your infidel temple to polytheism and the death of your so-called Supreme Pontiff, this man in white who you treat as though he were a god. This is your punishment for the sins of Iraq, Abu Ghraib, and Guantánamo Bay. The attacks will continue until the land of Iraq is no longer in American bondage and Palestine has been liberated from the clutches of the Jews. We are the Brotherhood of Allah. There is no God but Allah, and all praise to him.

Gabriel ran down the stairs, Angelli at his back.