Iran’s senior leaders had been planning for more than a week for an Israeli attack should nuclear talks with the United States fail. But they made one enormous miscalculation.

They never expected Israel to strike before another round of talks that had been scheduled for this coming Sunday in Oman, officials close to Iran’s leadership said on Friday. They dismissed reports that an attack was imminent as Israeli propaganda meant to pressure Iran to make concessions on its nuclear program in those talks.

Perhaps because of that complacency, precautions that had been planned were ignored, the officials said.

This account of how Iranian officials were preparing before Israel conducted widespread attacks across their country on Friday, and how they reacted in the aftermath, is based on interviews with half a dozen senior Iranian officials and two members of the Revolutionary Guards. They all asked not to be named to discuss sensitive information.

Officials said that the night of Israel’s attack, senior military commanders did not shelter in safe houses and instead stayed in their own homes, a fateful decision. Gen. Amir Ali Hajizadeh, the commander of the Revolutionary Guards’ aerospace unit, and his senior staff ignored a directive against congregating in one location. They held an emergency war meeting at a military base in Tehran and were killed when Israel struck the base.

By Friday evening, the government was just beginning to grasp the extent of damage from Israel’s military campaign that began in the early hours of the day and struck at least 15 locations across Iran, including in Isfahan, Tabriz, Ilam, Lorestan, Borujerd, Qom, Arak, Urmia, Ghasre Shirin, Kermanshah, Hamedan and Shiraz, four Iranian officials said.