CAIRO—Egyptian security forces arrested Mohamed Abdelsatar at the school where he worked as a teacher in April 2017. Another man, Sabry Salah, was arrested the next month.

Mr. Salah’s wife and Mr. Abdelsatar’s school asked the government for information about their whereabouts in the days after their arrests. None was forthcoming.

The month after each man disappeared, however, the government said they were dead—killed by security forces in exchanges of gunfire.

Those deaths and others like them are fueling allegations that Egypt’s security forces are killing detainees and later claiming they died in clashes with police, according to the accounts of Western security officials, victims’ relatives and documents seen by The Wall Street Journal.

The killings are among hundreds carried out by the Egyptian Interior Ministry as it wrestles with attacks by Islamic State and other militant groups. Those attacks have claimed the lives of hundreds of Egyptians in recent years. The militant attacks and the increasingly lethal response by security forces have resulted in more scrutiny on the human rights record of the Egyptian government, a key U.S. ally and the recipient of billions in American military aid.