Shortly before embarking on a fatal helicopter ride on Sunday, Iran’s president, Ebrahim Raisi, and his delegation of senior officials held a communal prayer. Someone suggested having lunch, but the president demurred, saying he was in a hurry to reach his next destination.

Mr. Raisi boarded the aircraft and sat by a window. The foreign minister, Hossein Amir Abdollahian, stopped for a picture with a crowd that was swarming the tarmac. He smiled and placed one hand over his chest while holding a brown briefcase in the other.

Around 1 p.m., a convoy of three helicopters took off from a helipad on Iran’s border with Azerbaijan, with the president’s craft in the middle. But about half an hour into the flight, the president’s helicopter vanished.

Calls to passengers on the president’s helicopter were met with silence until one answered. “I don’t know what happened,” Ayatollah Mohammad-Ali Al-Hashem said, sounding distraught. “I am not feeling well.” Two hours later, his phone, too, went silent.

As a frenzied 17-hour search unfolded, government officials began an aggressive effort to guard against possible threats from abroad and, especially, unrest at home, mindful of an uprising led by women and girls in 2022 that demanded the end of the Islamic Republic.