This massacre is a tragedy not just for the victims but for Egypt. It will act as a powerful recruiting sergeant for the sinister forces out to perpetuate the crisis.
The Muslim Brotherhood believes the security forces opened fire on demonstrators while they prayed. With a baffling sense of timing the authorities wheeled Hosnu Mubarak into a Cairo courtroom at the weekend.
He is charged in connection with the killings of demonstrators in 2011. By comparison no-one will be held accountable for the shootings during the current upheaval.
That means the security forces are unfettered and unrestrained. Retaliation for this morning's massacre will come. The first big car bombing or suicide attack can't be far off.
The Muslim Brotherhood tells ITV News they are against violence but will "pay the price of their blood" to restore their ousted president.
Egypt's army has named two further officials in the transitional government as its supporters and opponents vowed to stay on the streets.
There has been outrage in Egypt after 51 people were killed near an barracks where ousted President Mohamed Morsi is thought to be held.