Four police officers have been detained for four days pending investigations into the killing of 37 Morsi supporters while they were being transferred to prison in August.
On 18 August, the interior ministry confirmed 37 Morsi supporters had died of asphyxiation due to teargas and overcrowding while they were being transferred to Abu Zabaal prison in Cairo.
Security forces claimed the prisoners had died during an escape attempt. However, prosecution investigators said there had been no escape attempt.
Egyptian police have long been accused of using excessive force and torture. The January 2011 uprising, which led to the ouster of Hosni Mubarak, started as a protest against police brutality.