CAIRO — Dressed in military fatigues, the gunmen waved down the bus filled with Christian pilgrims as it wended its way down a dusty side-road in the desert of western Egypt, headed toward a monastery.

Claiming to be security officers, the gunmen ordered the passengers to get out. They separated the men from the women and children, and instructed them to surrender their mobile phones. They told the men to recite the shahada, the Islamic declaration of faith.

When the men refused, the gunmen opened fire.

At least 28 people were killed, several with a single shot to the head, according to the Egyptian authorities and relatives of the victims, several of whom were children. The attack on Friday in Minya Province, 120 miles south of Cairo, was a coldblooded escalation of sectarian violence targeting minority Christians that has left more than 100 people dead since December and shaken the country’s government.

There was no immediate claim of responsibility in the latest assault. Yet it bore the hallmarks of the Islamic State, which in the past six months has dispatched suicide bombers into crowded Sunday services and caused an entire community in northern Sinai to flee their homes in panic.