The government already has detained several top-ranking Brotherhood members, and many lower-ranking members worry that they, too, might face charges.
The case against Morsi arises from the tumultuous days in early 2011, when hundreds of thousands of Egyptians took to the streets to demand Mubarak’s resignation. Morsi, who was then a top Muslim Brotherhood official, had been arrested as the military tried to squelch the protests. He was held until armed men who may have been members of Hamas stormed into the prison and freed a number of prisoners.
Morsi was transferred Friday to the same prison where Mubarak is being held.
The prison break, which was part of the lore that had sprung up around Morsi, was declared illegal by a Cairo court on June 23, the same day that el-Sissi issued his first ultimatum for Morsi to reach an accommodation with his opponents or face likely military intervention.
In Washington, State Department spokeswoman Jen Psaki said Friday that the investigation into Morsi "deeply concerned" the Obama administration. She said the administration thought it was important "that there be a process to work towards his release. Clearly, this process should respect the personal security of him and take into account the volatile political situation in Egypt."
She noted that the U.S. thinks "it is very challenging to have an inclusive process if you have a number of officials from one party arbitrarily detained."
The military and police have enjoyed a resurgence of popularity since Morsi’s ouster even though the 2011 uprising began as a call against police brutality. Many who once protested against the police now praise them.
Police officers fatally shot Soha Sayed’s husband in 2011, but she now supports an Egypt governed by the security forces she once denounced.
“El-Sissi saved Egypt from the terrorist group,” the 43-year-old said, referring to the Muslim Brotherhood. “In any institution there are corrupt people. Not all of the police are bad.”
Lesley Clark contributed to this report from Washington.