Shaimaa El-Sabagh died from three birdshots wounds fired by an unidentified weapon, an official autopsy report has said.
El-Sabagh, who was killed at a protest in downtown Cairo on Saturday, died from heart and lungs lacerations and bleeding from the chest.
The shots that killed her were fired from eight metres away, the report added.
El-Sabagh, 33, was an advocate for workers' rights and a leading member of the Socialist Popular Alliance Party in Alexandria, a group which supported both the January 25 revolution and the June 30 anti-Brotherhood protests.
She was heading, with around two dozen marchers, to Tahrir Square to lay flowers in the memory of those who died during the 2011 uprising.