At least 36 people detained on suspicion of taking part in the street clashes roiling Egypt’s capital were killed on Sunday when security forces fired tear gas inside of the prison truck holding them, security officials said.

Those killed were part of a prison truck convoy of some 600 detainees heading to Abu Zaabal prison in northern Egypt, the 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 inside, the officials said. 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.

State media also said all those killed and the gunmen belonged to the Muslim Brotherhood, the organization that ousted President Mohamed Morsy 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 on Sunday night.

The officials who spoke to the AP said that the detainees were rounded up during the past two days of street violence around Cairo’s Ramses square, clashes that killed scores of people.

Egypt’s Muslim Brotherhood group is leading mass rallies against the country’s military over the July 3 coup that ousted Mr. Morsy. The Brotherhood, long banned in Egypt, rocketed to power in the country’s first democratic elections held last year.

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