A leading Socialist Popular Alliance Party (SPAP) member was detained pending investigation Saturday on "suspicion" of killing Shaimaa El-Sabagh, another leading party member who had been shot during a peaceful march in Cairo a week ago.
Zohdi El-Shamy, the vice president of the party, was considered a witness until the prosecution "suspected" he was involved in the killing El-Sabagh after questioning him, Al-Ahram Arabic site reported.
El-Shamy will be detained until investigations into these suspicions are complete on Sunday.
Prosecutors will then decide whether to release him or keep him in detention.
In a press conference right after her death, the SPAP accused the police of shooting dead El-Sabagh, describing the incident as a "premeditated murder."
The victim was taking part in a memorial march to Tahrir Square on 24 January to commemorate those killed in the 2011 uprising against former autocrat Hosni Mubarak.
The march, which was not authorised, was forcefully dispersed by the police who fired teargas and birdshot.
The interior ministry denied that a policeman had shot her dead.
The party said they will file a report to the prosecutor general Sunday asking to change the venue of the investigation's from the Qasr El-Nil prosecution due to its bias in favor of the police, the party's general secretary Talaat Fahmy told Reuters' Aswat Masriya.
Instead, the party has requested the appointment of an independent judge to conduct the investigations.
Fahmy added that Qasr El-Nil prosecution has turned El-Shamy from a witness to a defendant when he volunteered to retell what he witnessed during El-Sabagh's shooting.
He added that the prosecution didn't directly charge El-Shamy of killing El-Sabagh, but accused him of appearing suspiciously "anxious" in video footage of the march.
The SPAP and its leftist allies have threatened to boycott the upcoming parliamentary elections, scheduled to start in March, unless the government agrees to five demands.
These demands include sacking the interior minister as well as restructuring the police sector.On Thursday, dozens of women mourners held a loud vigil at the site of Shaimaa's murder to denounce her killers.