Updated

Egyptian security officials say suspected Islamist militants ambushed two minibuses in northern Sinai Monday, killing 25 policemen.

The officials say the attack took place as the two vehicles were driving through a village near the town of Rafah, located on the border of the Gaza Strip in the volatile Sinai Peninsula. According to authorities, the militants forced the two vehicles to stop and the policemen were ordered outside and made to lie down before they were shot execution-style. The officials said that the policemen were off-duty and in civilian clothes.

Initially, rocket-propelled grenades were believed to have been used in the attack. Egyptian state television also reported that the men were shot execution-style.

The officials told the Associated Press the attack also left two policemen wounded. The officials spoke on condition of anonymity as they were not authorized to talk to the media.

A few hours after the attack near Rafah, suspected militants shot to death a police major as he stood guard outside a bank in the city of el-Arish, also in northern Sinai, security officials said.

Sinai has been witnessing almost daily attacks by suspected militants since the July 3 ouster of Islamist President Mohammed Morsi in a military coup.

In another development Monday, an Egyptian public prosecutor ordered Morsi detained for 15 days, pending an investigation into allegations he participated in “violent acts,” according to state news agency MENA.

Reuters reports that Egyptian judicial authorities extended Morsi’s detention for 30 days last Thursday, in separate charges.

Morsi was accused of murder and spying and has been held at an undisclosed location since he was overthrown last month. The new case focuses on protests at the presidential palace last December, according to MENA.

Egypt's military and security forces have been engaged in a long battle against militants in the northern half of the peninsula. Militants and tribesmen have used the Sinai area for smuggling and other criminal activity for years, firing rockets into Israel and staging other cross-border attacks.

Word that former Egyptian President Hosni Mubarak could be released from custody later this week, raised concerns of more violence. Mubarak is on retrial for the killings of hundreds of protesters during the 2011 uprising that led to his ouster.

Monday’s killings came one day after police fired tear gas to free a prison guard from rioting detainees, killing at least 36. The deaths of the 36 detainees and the 25 policemen take to nearly 1,000 the number of people killed in Egypt since Wednesday's simultaneous assaults on two sit-in protest camps by Morsi supporters.

A U.S. State Department spokeswoman said Monday that the U.S. has not yet made a decision on whether it will freeze economic assistance to Egypt.

``We have not made a policy decision to put a blanket hold on the economic support fund,'' Jen Psaki told a press briefing Monday. She also said a review was underway which applied to economic, military and security assistance to Egypt.

At a White House press briefing Monday, spokesman Josh Earnest said the U.S. has made its position on the issue of assistance “crystal clear.” “The president has directed all of the relevant agencies and departments in this administration to review the assistance that's provided to the Egyptian government. That review is ongoing. And that review is being made in light of actions that are taken by the interim Egyptian government,” Earnest said.

The detainees killed on Sunday were in a prison truck convoy of some 600 prisoners heading to Abu Zaabal prison in northern Egypt, security officials told The Associated Press. Detainees in one of the trucks rioted and managed to capture a police officer inside, the officials said.

Security forces fired tear gas into the truck in hopes of freeing the badly beaten officer, the officials said. The officials said those killed died from suffocating from the gas. Those officials also spoke on condition of anonymity because they were not authorized to speak to journalists.

However, the officials' version of event contradicted reports about the incident carried by state media. The official website of state television reported that the deaths took place after security forces clashed with militants near the prison and detainees came under fire while trying to escape. The official MENA state news agency also said the trucks came under attack from gunmen.

State media also said the people killed and the gunmen belonged to the Muslim Brotherhood, the Islamist organization that Morsi hails from. The officials who spoke to AP said some of the detainees belonged to the Brotherhood, while others didn't. The differences in the accounts could not be immediately reconciled.

The government has ordered an investigation into the deaths.

Security forces arrested hundreds of Muslim Brotherhood members early Sunday in raids on their homes in different cities, aimed at disrupting planned rallies to support Morsi. The Cabinet also held an emergency meeting to consider banning the group.

A possible ban — which authorities say would be implemented over the group's use of violence — would be a repeat of the decades-long struggle between the state and the Brotherhood. It also would drain the group's financial resources and allow for mass arrests of its members. That likely would diminish the chances of a negotiated solution to the crisis and push the group again underground.

The Brotherhood has shown no signs of backing down.

Under the banner of an anti-coup alliance, the group held protests Sunday, though many appeared smaller in scale than others held in recent days.

"They think they can end the movement," said Muslim Brotherhood senior member Saad Emara. "The more killings, the more people join us."

The Associated Press and Reuters contributed to this report.