CAIRO, Egypt -- Egyptian authorities stopped the executive director of Human Rights Watch and another U.S. staffer from entering the country Monday ahead of the release of a critical report by the group on mass killings by security forces last summer, the group and security officials said.

This is the first time Egyptian authorities have stopped staffers from the New-York based group from entering the country.

Executive Director Kenneth Roth and Middle East and North Africa Director Sarah Leah Whitson were to brief diplomats and journalists on the findings of their investigation into the bloody events in July and August last year following the ouster of Islamist President Mohammed Morsi.

The report, which was to be launched Tuesday, documents the deaths of hundreds killed in six different incidents, including in one security raid on a sit-in of Morsi supporters at Cairo's Rabaah el-Adawiyah Square described by Human Rights Watch as the worst massacre in Egypt's modern history.

In a statement Monday, Human Rights Watch said Egypt's police and army "methodically opened fire with live ammunition," killing at least 1,150 protesters during the dispersals of the sit-in and five other demonstrations. No one has been held accountable for the crackdown and no formal investigation has been made public.

An airport official said the two were turned back on instructions from a security agency, without elaborating. The two had spent nearly 12 hours in Cairo International Airport, said the official, who spoke on condition of anonymity because he was not authorized to speak to reporters.

"It appears the Egyptian government has no appetite to face up to the reality of these abuses, let alone hold those responsible to account," Roth said in a statement.

Tweeting from the airport, Roth said the Rabaah "massacre was too ruthless for Egypt's head-in-sand approach to work. Barring HRW won't make the world forget."

Human Rights Watch said it had shared its findings with the government but received no reply. The group said it still plans to release its findings Tuesday, although a scheduled conference in Cairo is unlikely to take place. One of the report's main researchers, Omar Shakir, a U.S. citizen, also left the country Monday.

Shakir said the group had sent Egypt's Interior Ministry, Foreign Ministry, chief prosecutor and its ambassadors to Washington and the U.N. its findings during and after the investigation to give it time to share its versions of the events. But lacking a response, the group had to piece together the government's account from public and media statements, he said.

The government said at the time that the sit-ins -- in place for over 40 days -- were a threat to public order and that "terrorists" and armed gunmen were among the protesters. It blamed Morsi supporters of waging a violent campaign to destabilize the country, a charge denied by his Muslim Brotherhood group. Mediation efforts failed to convince Morsi supporters to call off the sit-ins, and a bloody crackdown on protesters in other parts of the city preceded the dispersal on Aug. 14.

The final toll was 624 dead, according to the semi-official National Council for Human Rights. Morsi supporters say they documented names of 2,500 dead, though the highest tallies by independent rights groups stay close to 1,000. At least eight policemen were killed during the dispersals.

The sit-ins' breakup set off violence in other parts of Egypt, when protesters and armed men attacked police stations, government offices and churches.

Interior Ministry spokesman Hani Abdel-Latif said he had no immediate response to the barring of the Human Rights Watch executives or the report. However, he said Egypt's National Council for Human Rights had conducted its own investigation into the dispersals.

"The Egyptian judiciary will have its say, and its decisions will be the ones to be implemented," Abdel-Latif said.

The Council had blamed Morsi supporters for shooting at police, escalating violence that ultimately led to the death of civilians, but it also held the security forces responsible for using excessive firepower and for failing to protect a safe corridor through which it intended to evacuate the protesters.

On Monday, the National Council for Human Rights renewed its call for an independent investigation into the events.

Human Rights Watch shut its offices in Cairo earlier this year because its request for registration that had been pending since 2007 was still unanswered. Shakir said that in light of "a shrinking space for civil society and a brutal crackdown against those who don't toe the government line," the group withdrew its permit application and closed its offices, fearing for its staff.

But Shakir said the group wanted to launch the report in Cairo to be part of the discussion regarding the bloody events of last year.

"It is unfortunate that the government, after putting out its own version of the events two days ago, decided to not allow an alternative narrative from being presented in Egypt," he said.