The army has sought to impose a tight seal on the area near the city of Sheikh Zuwaid, where the attacks occurred. In the last few months, it has resorted to destroying hundreds of homes near the border with the Hamas-controlled Gaza Strip in an attempt to restrict access to smuggling tunnels that might aid the militants.

Yet attacks on security forces have persisted. An assault on a military checkpoint in the area in October killed more than 30 soldiers and set off a hunt for what the Egyptian authorities believed was an army insider who had given the attackers information about troop movements. In January, a wave of several coordinated bombings across the region killed 44 people, according to reports in the Egyptian state news media.

Since January, however, attention has shifted to other groups of evidently less-seasoned militants who have sought to attack empty stores or other businesses in Cairo and across the country. Carried out by shadowy new groups with names like the Popular Resistance Movement and Revolutionary Punishment, the attacks typically used improvised explosives to strike electric utility stations or storefronts in an attempt to discourage investment in Egypt.

Banks and businesses tied to the United Arab Emirates or other Persian Gulf monarchies have been frequent targets, notably including Kentucky Fried Chicken fast food outlets operated by a Kuwaiti company.

One of the groups, Revolutionary Punishment, claimed responsibility this week for the killing of an off-duty security officer, Khaled Mohamed, who was leaving morning prayers at a mosque in his village in the province of Monofeya, reviving fears that the groups could turn to more lethal violence. The group said it had dedicated the attack to Islamist prisoners who had been sentenced to death and accused the officer of overseeing torture.