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Khalil Hamra, File, Associated Press
In this Tuesday, July 9, 2013 file photo, a supporter of ousted President Mohammed Morsi, with a national flag, gestures to army soldiers on guard at the Republican Guard building in Nasr City, Cairo. A year after hundreds were killed in the biggest massacre in modern Egyptian history, the division is even stronger, now drawn in blood, between Egypt’s traditional two strongest powers: A military aiming to restore its old order, and the Muslim Brotherhood trying to survive after being ousted from power. That rivalry likely locks Egypt into continued conflict, with fears democracy will be the loser.

CAIRO — Around 6:30 a.m., police armored vehicles rumbled up to the barricades at the edges of the anti-government sit-in where thousands of Islamists had camped out for weeks in a Cairo square.

First came tear gas. Then quickly, police started using machine guns. Every five minutes, student Mahmoud el-Iddrissi remembers, they swept the barricade with bullets. A friend next to him stood to throw a firecracker and immediately fell, shot in his neck and shoulder.

The scene on Aug. 14, 2013, was the start of the biggest massacre in modern Egyptian history, as security forces crushed the sit-in by Islamist supporters of Mohammed Morsi, the elected president who had been removed by the military a month earlier. At least 624 people were killed during 12 hours of mayhem in Cairo's Rabaah el-Adawiyah Square, though rights groups have said the toll may be several hundred higher.

An Associated Press investigation shows that commanders gave security forces virtual carte blanche to use deadly force. Authorities contend police only responded with live ammunition on anyone who fired on them — and eight policemen were killed by gunmen during the assault.

But broad orders to the security forces, revealed to AP, emphasized crushing resistance. Police were told to expect to come under fire and to move strongly to eliminate any threat. They were also told no one would be prosecuted for any deaths, two generals in the Interior Ministry, which is in charge of police, told the AP. The generals spoke on condition of anonymity to discuss the planning.

The investigation also showed both the military-backed government and Morsi's Muslim Brotherhood, the main force behind the protest, staunchly resisted any concessions that international mediators hoped could have averted the disaster. The AP interviewed more than 20 surviving protesters, security officials and diplomats.

A year later, the division is even stronger, now drawn in blood, between Egypt's traditional two strongest powers: A military aiming to restore its old order, and the Muslim Brotherhood trying to survive after being ousted from power. That rivalry likely locks Egypt into continued conflict, with fears democracy will be the loser.

The Brotherhood did not go quietly. Over the next month, its sit-in protest at Rabaah el-Adawiyah Mosque grew. Morsi supporters camped out in hundreds of tents. From a stage at the center, prominent clerics and other figures daily told crowds they would hold out until Morsi was restored, denouncing el-Sissi and the military leadership as traitors fighting a war against Islam. A second, smaller sit-in was located across the capital in Nahda Square.

Tensions swelled. Security officials called Rabaah a threat that must be dealt with, saying armed "terrorists" were among the protesters. Rights groups have since confirmed that there were a few with automatic weapons, but that it hardly constituted an "armed camp" as officials depicted. Twice, police fired on protesters who marched out of the camp, and more than 100 were killed.

The protesters, in turn, contended they were standing up for democracy and vowed never to recognize a government installed by what they called a coup. Their protest failed to garner wider public support, but the Brotherhood and its allies increasingly pinned their cause on an all-or-nothing stand in Rabaah.

Those entrenched stances doomed mediation attempts led by four envoys: U.S. State Department official Robert Burns, EU envoy Bernadino Leon, and diplomats from Qatar, an ally of the Brotherhood, and the United Arab Emirates, an ally of Egypt's military.

On Aug. 4, the diplomats met in prison with the Brotherhood's most powerful man, deputy leader Khairat el-Shater, arrested soon after Morsi's ouster. As the group's top decision-maker, el-Shater's say was crucial.

El-Shater was willing to have a dialogue but only "between equals" — meaning prisoners must be released, Leon said. El-Shater suggested that Saad el-Katatni, a senior Brotherhood figure, should be the first freed. El-Shater recognized that he and Morsi would not be immediately released, according to Leon, giving the AP his first detailed account of the mediation.

It seemed a good sign. Diplomats had already proposed to the government that it release el-Katatni and Brotherhood-allied politician Aboul-Ela Madi to act as negotiators.

But in Rabaah, Brotherhood leaders and their allies adhered to their stance that Morsi must be freed.

"Any dialogue must be with the legitimately elected president," Gamal Abd-ul-Sattar, a senior figure in the Brotherhood-led anti-coup alliance, told the AP in response to questions sent to the Brotherhood's press office in London.

Leon said each side demanded the other act on reducing tensions, but neither did. Authorities wanted an end to rhetoric from Rabaah they saw as inciting violence. The protesters pointed to media portraying them as terrorists.

Protest leaders were convinced that concessions were useless and that the military was determined to crush the Islamist movement.

"I do not believe anything could have averted them as they were intent on stifling any dissent," Abd-ul-Sattar said. And protesters themselves did not want to back down, he said, adding that with the assault imminent, sit-in organizers asked women to leave the protest but they refused.

At the same time, a prominent ultraconservative Salafi cleric, Sheik Mohammed Hassan, was secretly mediating with both sides. In an Aug. 3 sermon, Hassan announced that the military promised not to break up the sit-ins if protesters ease their rhetoric and not leave the square in marches.

Furious that talks were revealed, Rabaah leaders denounced him. One of them, Salah Sultan, vowed never to recognize the post-Morsi government, calling el-Sissi "the traitor, the murderer, the liar."

The international envoys presented their proposal: The government would release some Brotherhood leaders, while the protesters would reduce numbers in the squares and tone down rhetoric. International experts would investigate claims of weapons among the protesters and of violence on both sides.

Each side replied, "It is the other side who should start first," Leon said.

Things were falling apart. U.S. Senators John McCain and Lindsey Graham arrived to meet with both sides. Their 45-minute sit-down with el-Sissi on Aug. 6 quickly soured. They pressed him to release detainees, while he insisted the jailed Brotherhood leaders had committed crimes.

El-Sissi grew "very agitated" and said "order must be restored," McCain told the AP. "The meeting came to a rather abrupt end."

After the meeting, McCain told a press conference that he considered Morsi's ouster a coup, further angering the government.

The next day, the government announced that mediation had failed.