Egyptian officials investigating the massacre of pilgrims at a mosque in Sinai say the attackers carried the flag of the Islamic State group.
At least 305 people were killed in the attack, which broke out during Friday prayers and no group has yet claimed responsibility.
Egypt's prosecutor says up to 30 assailants have arrived at the scene. President Abdul Fattah al-Sisi has vowed to respond with "maximum force".
The Egyptian army says it has already carried out airstrikes on "terrorist" targets.
Friday's attack on Bir al-Abed is the deadliest in the country's recent memory.
The Al-Rawda mosque was bombed and then dozens of gunmen, waiting outside, opened fire on those trying to escape. Some of the attackers wore masks and military uniforms.
The attackers reportedly set fire to parked vehicles to block access to the building and fired at ambulances.
Thirty children are among the dead and more than 100 people have been injured.
The country is in heavy mourning
Church bells rang today across Egypt, for the 305 victims of the largest terrorist attack in the country's modern history.
The Coptic Church took part in the heavy three-day national mourning of Egypt today, with all its temples ringing its bells throughout the territory at exactly 12 noon. An act of solidarity, particularly emphasized by the local media.
At the same time, in the churches people were praying and countless are the Christians who lit a candle in memory of all these innocent victims. The Coptic Church of Egypt expressed its warmest condolences to the families of the victims and wished a speedy recovery to the injured.