Some 36 Muslim Brotherhood supporters have died in disputed circumstances as unrest continues in Egypt.
The Interior Ministry said the detainees had tried to escape from a prison on the outskirts of Cairo on August 18.
It said the detainees suffocated when police fired tear gas to free an officer who had been taken hostage.
Other reports, however, said the Muslim Brotherhood supporters had died from asphyxiation in the back of a crammed police van while being transferred to prison.
The incident comes after the leader of Egypt's army warned that he will not tolerate further violence.
In a statement on August 18, General Abdul Fattah al-Sisi said, "We will never be silent in the face of the destruction of the country."
He also struck an apparently inclusive note, telling Morsi's supporters that "there is room for everyone in Egypt."
Latest government figures show at least 830 people have been killed since August 14 in clashes between backers of deposed President Muhammad Morsi and security forces.
Meanwhile, Saudi Foreign Minister Saud al-Faisal warned the West against pressuring Egypt's military to end the crackdown after talks on August 18 in Paris with French President Francois Hollande.
"We spoke about common views about what is going on," Faisal said. "We hope that common views would lead to a united stand about what needs to be done. I do not think that pressure on Egypt would work or help in restoring peace and national security in Egypt."
Riyadh was a close ally of Egypt's former leader Hosni Mubarak, toppled by a popular uprising in 2011 that brought Morsi's Muslim Brotherhood to power.
Hollande called for fresh elections in Egypt.
"We have a common responsibility -- Arab countries, Europe, France -- to work so that the violence stops, so that Egyptian political authorities can take the road map as a reference for the coming weeks, and to allow the organization of elections quickly so that the Egyptian people can express their point of view," the French president said.
In a rare joint foreign policy statement, the presidents of the European Commission and the European Council said it's the responsibility of the army and the interim government to end the violence.
EU foreign ministers meet in Brussels this week to review what steps to take.
Meanwhile, at least 24 Egyptian police officers have reportedly been killed in an ambush in the volatile Sinai Peninsula.
Security officials said the police were attacked by armed men near Rafah on the Gaza border.
Militants have stepped up attacks in Sinai, mainly targeting the Egyptian security forces, since the military overthrew and detained Morsi.
With reporting by AP and Reuters