Underscoring the growing anger over Mohammed Morsi’s ouster, suspected Islamic militants ambushed two minibuses carrying off-duty policemen in Egypt’s Sinai Peninsula, forcing the men to lie on the sand and shooting 25 of them dead.

“They were marked in advance by the attackers,” said Ashraf Abdullah, who heads the police branch the victims belonged to. He said the assailants checked the IDs of the men, who were not in uniform, to ensure they were policemen before opening fire.

The brazen daylight attack raised fears that the strategic desert region bordering Israel and the Gaza Strip could be plunged into a full-fledged insurgency.

The 25 slain police officers were given a funeral with full military honors presided over by Interior Minister Mohammed Ibrahim, who is in charge of the police, and the army’s chief of staff, Gen. Sedki Sobhi.

In a show of solidarity, the men’s coffins, draped in red, white and black Egyptian flags, were jointly carried by army soldiers and policemen, and interim President Adly Mansour declared a nationwide state of mourning to mark their deaths.

Despite the violence, Cairo, a bustling metropolis of some 18 million people, began to regain a sense of normalcy although the capital remained under a state of emergency and a dusk-to-dawn curfew.

Daytime traffic was back to its normal congested levels and stores were open. Government employees returned to work and the Central Bank ordered banks, which were operating on a reduced 9 a.m.-noon schedule, to remain open for an additional hour today. A handful of protests erupted in various parts of the city, but they were small and led to no violence.

Mubarak, 85, has been in detention since April 2011, two months after he was ousted in a revolution against his rule.

Last week, the military raided two protest camps of Morsi’s supporters in Cairo, killing hundreds of people and triggering a wave of violence that has left at least 1,000 people dead.

Monday’s attack targeting the police officers took place near the border town of Rafah in northern Sinai.

A few hours later, militants shot to death a senior police officer as he stood guard outside a bank in el-Arish, another city in the largely lawless area, security officials said.

There was no immediate claim of responsibility for either attack.

The United States condemned the slaying of the police officers.

U.N. Secretary-General Ban Ki-moon also denounced the attack.

“The Sinai Peninsula remains an area of concern, and the current situation in Egypt has not improved the situation,” State Department spokeswoman Jen Psaki told reporters in Washington. “The United States continues to support Egypt’s ongoing efforts against terrorism and growing lawlessness in the Sinai, and we continue to cooperate with Egypt in these efforts.”

The attacks came a day after security forces killed 36 detainees during a riot on a prison-bound truck convoy north of Cairo. The killings came as police fired tear gas to free a guard who was trapped in the melee, security officials said.

On Monday, the government ordered an inquiry into the deaths, which it blamed on armed men allegedly trying to help the 600 Muslim Brotherhood detainees escape. It gave no details.

The Brotherhood blamed military chief, Gen. Abdel-Fattah el-Sissi, and the interior minister for Sunday’s killings. The group also called for an international inquiry into the deaths.

The United States said it was troubled by the “suspicious deaths” of the prisoners.

“We call on all Egypt’s leaders and the international community to condemn such attacks without equivocation,” said Psaki, the State Department spokeswoman.

Amnesty International demanded a “full, impartial and effective” probe into the events.

Morsi has been held in an undisclosed location since his ouster. On Monday, prosecutors ordered his detention for 15 days in connection with allegations that he conspired to kill and torture protesters during mass demonstrations by the opposition outside his presidential palace in December 2012.

Associated Press writer Deb Riechmann contributed to this report from Washington.