Prosecutors ordered on Sunday the release of a leading Socialist Popular Alliance Party (SPAP) member who was detained one day earlier on "suspicion" of involvement in the killing Shaimaa El-Sabagh.
Shaimaa El-Sabagh, another leading party member of the SPAP, was killed during a peaceful march on 24 January to commemorate the martyrs of the 25 January revolution on its fourth anniversary near Tahrir Square in Downtown Cairo.
Zohdi El-Shamy, the vice president of the party, was released by prosecutors from custody after investigations carried out by the National Security Apparatus cleared him, Ahram's Arabic news website said.
El-Shamy had come forward as a witness to testify on the events that occurred during the SPAP's memorial march.
The march, which was not authorised, was forcefully dispersed by the police who fired teargas and birdshot.
Shaimaa El-Sabagh, the 33-year-old activist, was shot three times during the dispersal by birdshot, according to the Forensic Medicine Authority's official autopsy report.
El-Sabagh's death created a public outcry in Egypt.
The SPAP accused the police of the "premediated murder" of El-Sabagh.
The interior ministry denied that a policeman killed Shaimaa.
The SPAP and its allies from parties in the Pro-25 January Revolution Democratic Current 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 killing.