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CAIRO, Egypt — It was just before dawn on July 8 when guards protecting a sit-in supporting ousted President Mohamed Morsi reported seeing military and police forces approaching from side streets.

The security forces fired tear gas and blanks into the air, above the sit-in where worshippers had gathered to perform prayers. What followed next, according to one witness, “was the chaos of nightmares.”

The subsequent killing of 51 Morsi supporters outside the Republican Guard headquarters in northeast Cairo in the early morning hours of July 8 marked the single bloodiest state-led massacre since Egypt’s 2011 uprising.

Egyptian military spokesman Col. Ahmed Ali says the army acted in self-defense after violent protesters attacked the facility on motorbikes.

But a GlobalPost investigation, based on eyewitness testimonies, visits to the scene, interviews with morgue and military officials, and an examination of video and photographic evidence, paints a different picture.

Instead, the scene that emerges is one of a pre-planned attack on largely unarmed civilians, including arbitrary arrests and the complicity of some health facilities.

It aligns with the conclusions of international human rights groups also investigating the incident — that security forces used intentional lethal force against protesters, according to London-based Amnesty International.

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"They started to shoot people at the fence, then they started to shoot people further back,” said Hamza Mamdouh, a computer programmer and protester who witnessed the attack. “A man fell down beside me. They [the security services] were controlling everything."

The sit-in began in the wake of the July 3 military coup, bolstered by unprecedented street protests calling for Morsi’s ouster beginning on June 30.

Morsi supporters gathered outside the officers' club of the Republican Guard, an armored division responsible for protecting the capital, where they still believe their leader is being held incommunicado.

They have vowed not to leave until their president, a Muslim Brotherhood leader, is reinstated. He served just one year in office.

It was five days after the military seized power when, shortly after 3:30 a.m., the pro-Morsi protest’s informal security guards report seeing police and army troops approaching their sit-in from streets on their eastern and western flank.

As the crowd prayed, the guards began hitting lampposts and iron gates to create noise — a way of sounding the alarm ahead of attacks that was also used in Cairo’s iconic Tahrir Square during the 2011 revolution.

Some remained on the ground, finishing their prayers as quickly as possible, according to witnesses. Many others ran: some attempting to flee, others choking through the tear gas to find relatives in the protest tents that lined the road.

According to demonstrators and local residents, a deadly street battle of gunfire, Molotov cocktails and stones then unfolded over the next four hours, leaving 51 protesters and three security service members dead. More than 650 pro-Morsi supporters and bystanders were arrested in sweeping raids.

It is unclear which side fired the first shot of live ammunition in the pre-dawn darkness.

One Egyptian journalist, Mirna El Helbawi, live-tweeted the event from her balcony overlooking the protest. In a later post on Facebook, she says protesters fired the first bullets.

According to video footage, protesters fired at least three handguns. But these instances took place after sunrise, or several hours following the start of the battle.

Eyewitnesses interviewed by GlobalPost, including protesters and local residents, unanimously report that military troops used live ammunition while police fired birdshot at the gatherers.

Pro-Morsi protesters also carried handfuls of used live rounds, many carrying the insignia of the Egyptian army.

Lampposts and cars along the road where the battle took place are also pockmarked with bullet holes, the angle of which suggested they had been fired from above. This corroborates witness statements that military snipers fired into the crowd from rooftops and high windows.

One photojournalist captured his own death on film. As Ahmed Assem, a photographer for the Muslim Brotherhood's newspaper, trained his lens on the sniper atop a nearby building, the khaki-clad figure turned his rifle to the cameraman. The footage then ends abruptly. By lunchtime, Assem's body lay in a Cairo morgue. He had been killed by a single shot to the head.

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“They were shooting at us from different places: from behind the fence, from the Republican Guard,” said Ahram Abdel Aziz Gharib, an assistant professor of microbiology at Zagazig University, in a province outside Cairo.

“As I lay on the ground, I was shot with birdshot in the right side of my back — and started to cough up blood,” Gharib said from one of the local field hospitals, clutching the x-rays that document the injuries.

Her skeleton was peppered with birdshot, 75 pellets in total.

As casualties like Gharib were moved from makeshift field hospitals to medical facilities across Cairo, witnesses said a number of the wounded were repeatedly turned away from state-run hospitals.

At Cairo’s al-Ta’min al-Sihi hospital, patients reported to Amnesty International researchers that they struggled to procure medical reports from the facility.

Relatives at Cairo's Zeinhom morgue also struggled to retrieve the bodies of their loved ones. In some cases, the facility refused to release the bodies unless the families signed certificates stating that they had died of natural causes, according to journalists at the scene.

A senior pathologist at Cairo’s central morgue said his facility was “overwhelmed” by the number of corpses arriving throughout the day of July 8. He spoke on the condition of anonymity.

By the time evening fell, he said his staff had received at least 54 bodies, many with gunshot wounds to the head, back or chest. GlobalPost saw photos taken by morgue staff that corroborated his testimony.

“We believe it was the work of snipers,” the pathologist said, referring to the military's gunmen. “I worked in this morgue throughout the revolution … but this was our saddest day.”

The military has said it maintains the right to defend itself, even as organizations like Human Rights Watch and the United Nations call for independent inquiries into the massacre.

“When you are in danger, it is a soldier’s right to defend himself,” Col. Ahmed Ali said.

His claim that 15 motorcyclists attacked the facility, prompting soldiers to respond, could not be verified either by local residents or video footage of the clashes.

Military representatives say there is currently no open investigation into the killings.

Egypt’s interim president, Adly Mansour, has ordered an investigation by a civilian “judicial panel.” US Deputy Secretary of State William Burns met with Mansour in Cairo Monday, and warned the military-appointed government to halt its crackdown on political opponents.

"We have called on the military to avoid any politically motivated arrests," Burns said in a speech in Cairo Monday. "And we have also called upon those who differ with the government to adhere to their absolute obligation to participate peacefully.”

But the military, under the new constitutional declaration, has the right to investigate its own personnel through a separate justice system.

“We have seen again and again how Egypt’s military justice system cannot investigate serious human rights abuses with any impartiality,” said Joe Stork, acting Middle East director at human Rights Watch.

“Military prosecutors and judges remain in the same line of command as those they are investigating,” he said. “Making independence and impartiality impossible.”

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