That many Sunnis would prefer to take their chances under a militant group so violent it was thrown out of Al Qaeda sharply illustrates how difficult it will be for the Iraqi government to reassert control. Any aggressive effort by Baghdad to retake the city could reinforce the Iraqi Army’s reputation as an occupying force, rather than a guarantor of security.

Many of those who fled said they were terrified of possible airstrikes and indiscriminate shelling that they have seen, in news reports, against insurgents in Sunni-dominated Anbar Province, which has been out of government control for more than six months. Some, saying a rumor had been swirling through the local population, even worried that the Americans would be back to bomb their city. And most said the militants in Mosul had not terrorized the population and were keeping a low profile, with a small number of men in black masks staffing checkpoints.

“We are afraid it will be the same situation as in Falluja and Ramadi,” said a municipal worker who gave his name only as Abu Mohammed, for fear of losing his job. He was referring to the two cities in Anbar that have borne the brunt of government airstrikes, which have killed hundreds of civilians.