CAIRO — Her friends wanted to lay a wreath in Tahrir Square as a memorial, but Shaimaa el-Sabbagh urged them to reconsider. She feared that the police might attack, mistaking them for supporters of the outlawed Muslim Brotherhood, said a cousin, Sami Mohamed Ibrahim.

But how, her friends asked, could the police attack civilians who were armed only with flowers? So she kissed her 5-year-old son, Bilal, goodbye; left him in the care of a friend near her home in Alexandria; and, a day before the anniversary of the start of the Arab Spring revolt here, boarded a train for Cairo.

By midafternoon on Jan. 24, Ms. Sabbagh, 31, lay dead on a crowded street downtown, a potent symbol of the lethal force the Egyptian authorities have deployed to silence the cacophony of protest and dissent unleashed here four years ago. Human rights advocates say the cold brutality of her killing shows how far the military-backed government is willing to go to enforce a return to the old authoritarian order.

Stark images of her killing resonated so widely here that in a televised appearance Sunday, even President Abdel Fattah el-Sisi offered condolences, declaring that he saw Ms. Sabbagh as “my own daughter.” At the same time, police attempts to deflect blame for her killing have been undercut by Ms. Sabbagh’s personal profile: as a mother, an accomplished poet and a left-leaning activist who supported the military ouster of the Islamist president.