Bodies are laid out in a makeshift morgue on Aug. 14, 2013, after Egyptian security forces stormed two huge protest camps in Cairo. (Mosaab el-Shamy/AFP/Getty Images)

Egyptian security forces likely committed crimes against humanity by carrying out mass killings of anti-government demonstrators in Cairo last summer, New York-based Human Rights Watch said in a report released Tuesday.

The report also called for former military chief Abdel Fatah al-Sissi — now Egypt’s president — to be investigated for his role in the killings.

The 188-page dossier details the findings of a year-long investigation into the violence surrounding the military’s ouster of President Mohamed Morsi on July 3, 2013. It alleges that Egyptian police and army forces systematically killed unarmed protesters opposed to the coup as part of a policy devised at the highest levels of the government.

In a statement Tuesday, the government’s State Information Service, a public relations portal, called the report “negative and biased.” Human Rights Watch committed “outrageous interference” in the government’s own investigations by releasing the report, the statement said.

According to Human Rights Watch, Egyptian authorities presided over “one of the world’s largest killings of demonstrators in a single day in recent history” when they unleashed security forces to disperse two large pro-Morsi protest camps in Cairo on Aug. 14, 2013, leaving about 900 people dead.

The report calls on U.N. member states to investigate Sissi and at least nine other senior security officials under the principle of universal jurisdiction, a doctrine that enables national courts to prosecute individuals connected to serious crimes committed anywhere in the world. One of those recommended for investigation is Mohamed Farid el-Tohamy, the head of Egypt’s General Intelligence Service, who met with U.S. Secretary of State John F. Kerry in Washington in April.

The report’s release comes two days after officials at Cairo International Airport barred two senior Human Rights Watch executives from entering Egypt, on grounds that they were coming to “work illegally,” Egypt’s Interior Ministry said Tuesday. Kenneth Roth, executive director of Human Rights Watch, and Sarah Leah Whitson, the organization’s Middle East and North Africa division chief, had been set to brief journalists and diplomats on the report’s key findings in Cairo on Tuesday. Instead, both held a videoconference from abroad after accusing the government of stifling dissent. The organization closed its office in Cairo in February because of increasing restrictions on civil society, the group said.

“It appears the Egyptian government has no appetite to face up to the reality of these abuses,” Roth said in a statement Monday.

“The Rabaa massacre is too big to go away, and the public will not forget,” Roth said via videoconference from New York on Tuesday. He was referring to Rabaa al-Adawiya Square, site of the largest protest encampment attacked last August.

Among the inquiry’s key findings — based on on-site investigations, video footage, government statements and interviews with more than 200 witnesses — is that the deaths of more than 1,000 protesters in six incidents from July 5 to Aug. 16, 2013, were part of a government policy “to attack unarmed persons on political grounds” and that they therefore “constitute crimes against humanity,” Human Rights Watch said.

According to the International Criminal Court, crimes against humanity include violent acts committed as part of a widespread or systematic attack directed against a civilian population. Human Rights Watch says Egyptian security forces “systematically and deliberately killed largely unarmed protesters on political grounds — those perceived to be affiliated or sympathetic with the Muslim Brotherhood and opposed to the July 3 ouster” of Morsi, a Brotherhood stalwart.

A press officer at the Interior Ministry, which Human Rights Watch alleges is responsible for the vast majority of the killings, declined to comment on the report’s findings when reached by telephone Monday. Requests for comment from the president’s office also were unanswered Tuesday.

Egypt’s government has long maintained that demonstrators at the two camps — at Rabaa al-Adawiya and al-Nahda squares in Cairo — were armed and that lethal force was employed as self-defense against gunmen at the sit-in. Human Rights Watch, however, says that very few of the tens of thousands of demonstrators were armed and that the government response “went far beyond the bounds permitted by international human rights law.”

According to a Human Rights Watch arms researcher who reviewed footage of the 12-hour operation to disperse the Rabaa protesters, government snipers who were perched on rooftops surrounding the square were professionals “carrying 7.62 mm bolt action rifles equipped with optics and supported by armed spotters using binoculars.”

Forensic evidence reviewed by Human Rights Watch shows that most of the deaths among protesters on Aug. 14 were from gunshot wounds to the head, neck and chest. Six witnesses told Human Rights Watch researchers that they saw security forces beat, torture and summarily execute some of the 800 protesters who had been detained. It is also likely that police set the fires that engulfed the nearby Rabaa hospital and field clinic, the report says.

The subsequent government effort to conceal the events has thwarted attempts by rights groups to establish the actual death toll, and no police or army officer has been held accountable for the killings, Human Rights Watch says.

Activists here have accused the government of fostering an environment of impunity for security forces and officials involved in abuses, maintaining one of the key characteristics of the regime of Hosni Mubarak, who was ousted in 2011. As defense minister, Sissi toppled Morsi, the country’s first democratically elected president, after mass protests against his brief rule. Sissi was then elected president in May and has overseen one of the most repressive periods in Egypt’s modern history, rights advocates say.

“Until now we do not know how many people died in the dispersal [at Rabaa]. We are dealing with a government that lacks transparency,” said Ahmed Osman, a lawyer with the Cairo-based Association for Freedom of Thought and Expression.

“They think everything they have done is according to the law,” Osman said. “So how can you hold anyone responsible if there is no crime?”

Sharaf al-Hourani contributed to this report.