Authorities in Egypt have barred two senior executives of Human Rights Watch from entering the country to unveil a year-long investigation of mass killings of anti-government demonstrators last summer at the hands of security forces.

Officials at Cairo International Airport refused entry to Kenneth Roth, executive director of Human Rights Watch, and Sarah Leah Whitson, the New York-based group’s director for the Middle East and North Africa, without an official explanation, Human Rights Watch said Monday.

“This is the first time that Egyptian authorities have denied Human Rights Watch staff members entry to the country, including during the Mubarak government,” the group said in a statement, referring to ousted president Hosni Mubarak.

Roth and Whitson were scheduled to brief journalists and diplomats Tuesday on a 188-page report outlining the role of the police and the army in the systematic killing of more than 1,000 demonstrators who were protesting a military coup against elected President Mohamed Morsi last summer.

Also Monday, Egypt’s public prosecutor launched an investigation of imprisoned democracy activist Ahmed Maher for alleged high treason over a June 4 op-ed piece he wrote for The Washington Post from Tora prison. Maher is serving a three-year sentence for violating Egypt’s protest law, which bans unauthorized public gatherings of more than 10 people.

The op-ed, titled “What really happened to the Egyptian revolution,” lambasted Secretary of State John F. Kerry for praising Egypt’s government amid widespread human rights abuses. A suit filed by an Egyptian lawyer accuses Maher of calling on the U.S. government to interfere in Egypt’s internal affairs, according to a statement from the prosecutor’s office. The crime of high treason carries the death penalty in Egypt.

Maher is a founder of the April 6 Youth Movement, which helped spearhead the uprising against Mubarak in 2011. Egypt’s government has arrested tens of thousands of people in a far-reaching crackdown on dissent since Morsi’s July 2013 ouster, and many prominent figures who helped topple Mubarak also have been jailed.

“This is blatant revenge by the regime over the revolution that I and other members of the April 6 Youth Movement had the honor of helping to spark in 2011,” Maher wrote of the mass arrests in a separate Post op-ed piece published Feb. 7.

According to Human Rights Watch, “Authorities have imposed extensive restrictions on civil society organizations over the past year.”

The group closed its office in Cairo in February “because of concerns about the deteriorating security and political environment in the country,” it said in its statement Monday.

Sharaf al-Hourani contributed to this report.