This post was updated 1:00 a.m. ET Monday
The Associated Press reports the suspects killed were part of a prison truck convoy of some 600 detainees heading to Abu Zaabal prison in northern Egypt. Detainees in one of the trucks rioted and managed to capture a police officer inside.
Security forces fired tear gas into the truck in hopes of freeing the badly beaten officer, security officials told the AP. The officials said those killed died from suffocating on the gas.
The officials spoke on condition of anonymity because they were not authorized to speak to journalists.
(Post was updated at 3:50 p.m. ET Sunday)
Egypt's army kept several large squares in Cairo locked down on Sunday after days of bloody confrontations between security forces and supporters of ousted President Mohammed Morsi.
Security was also beefed up at a key courthouse in the capital after the Muslim Brotherhood, under the banner of an anti-coup alliance, vowed to stage a mass demonstration there in support of Morsi.
Journalist Sharif Abdel Kouddous said he thinks there will be more violence on Sunday as a result of the planned rallies.
Kouddous, speaking to Weekend Edition Sunday host Rachel Martin, says "both sides are vowing to escalate."
Gen. Abdel Fattah el-Sisi, Egypt's deputy prime minister, gave a televised speech on Sunday that sounded vaguely conciliatory.
"There is room for everyone in Egypt," he said, adding however that the country would never bend to violent acts.
On Saturday, Egyptian forces stormed the Fateh Mosque, where members of the Muslim Brotherhood had taken refuge. Security sources tell NPR that 36 were killed in the clash at the mosque and state news agency MENA says a total of 79 people were killed and 549 wounded across the country in violence on Saturday. The news agency, quoting government sources, said 830 people had died in clashes since a nationwide crackdown began on Wednesday.
Police also arrested the brother of al-Qaida chief Ayman al-Zawahri accusing him of plotting to bring in armed groups to support for anti-government forces. Mohammed al-Zawahri, a Morsi ally, is the leader of the ultraconservative Jihadi Salafi group which espouses al-Qaida's hard-line ideology, The Associated Press says.
The Egyptian government is also reportedly mulling a ban on the Muslim Brotherhood in an effort to quell the demonstrations.
The AP reports that in the days since the crackdown began, Islamists have attacked dozens of Coptic churches as well as homes and businesses owned by Christians. The news agency said Sunday that a Franciscan school in the capital was torched and that three nuns had been "sexually harassed and abused as they fought their way through a mob."
Rev. Mikhail, a pastor at a church in Alexandria who asked that his surname not be given, describes to NPR the attacks on "Christian churches, Christian schools, Christian bookshops and even Christian orphanages" in recent days.
Mikhail told WESUN that the attacks varied in their severity and that most had been in the country's south.
He blames a long-standing "anti-Christian rhetoric" among Islamists, but also a belief that Christians had a lot to do with removing Morsi from power.
"Christians did play a part, but a small part" in Morsi's removal, Mikhail says.
Meanwhile, Congress is split over the issue of whether the U.S. should cut off aid to Egypt, with Sen. John McCain (R-Ariz.) among those who have called for an end to the flow of money in light of the July 3 coup against Morsi and the subsequent crackdown on his supporters.
Speaking on PBS' Washington Week with Gwen Ifill, McCain said the U.S. is "violating our own rule of law" by continuing aid.
"Because our law clearly states that if it's a military coup then aid is cut off. They had the coup, and then of course we didn't do that. That's a blow to credibility," McCain said.