As Mr. Mitri sat talking to a reporter in his home, he received a text message from the kidnappers, who were demanding a ransom of 150,000 Egyptian pounds, about $21,500. “They want to know if I have the money in the house with me,” he said, his face creased with anger and fear.

The military announced last week on its Facebook page that its counterterrorism operation over the past month in Sinai had led to 103 arrests, and the destruction of 102 tunnels, 40 underground fuel storage tanks and 4 houses used by extremists. The reference to tunnels, presumably those used to smuggle weapons and goods into Gaza, tallies with the military’s routine suggestions that Hamas, the Palestinian militant group that controls Gaza, is involved in the violence. However, there is no evidence to suggest that is true.

Most residents here say the authorities appear to be on the defensive, with soldiers and the police hunkered down at their posts and suffering daily casualties.

Although the attackers mainly hit the police and the military, a dozen civilians have been killed in the cross-fire and many others have been wounded, according to local hospital officials. Hanny Aish, a worker at a cement factory, was on a bus in the wee hours of July 15 when a rocket-propelled grenade crashed through the window, killing 3 of the 20 people on board and wounding the rest.

“I was sitting behind the driver, and all of a sudden I had flesh and blood all over me,” Mr. Aish, who suffered glass cuts across his body and shattered eardrums, said during an interview at his home.

The attackers’ target appears to have been an armored vehicle that passed by just afterward. But as in the rest of Egypt, even the most basic perceptions of the conflict reflect a bitter division between supporters and opponents of Mr. Morsi. The first group tends to blame the military for all the recent attacks here, saying it is staging fake jihadist attacks to build a pretext for a future crackdown.