Gunmen were seen flashing an Islamic State flag during their attack at a mosque in Egypt’s North Sinai province on Friday, which killed 305 people, the country’s public prosecutor said on Saturday. No group has claimed responsibility for the attack yet.

“The worshippers were taken by surprise by these elements [the gunmen],” the prosecutor said in a statement, as reported by Reuters. “They numbered between 25 and 30, carrying the Daesh [Islamic State] flag and took up positions in front of the mosque door and its 12 windows with automatic rifles.”

The prosecutor’s office is interviewing injured people as part of its investigation. The attack was earlier said to resemble those by an affiliate of the Islamic State active in the region.

Apart from those killed, at least 128 people were injured after militants set off a bomb and opened fire at the Al Rawdah Mosque in Bir al-Abed, around 50 km west of North Sinai Capital El-Arish. The masked gunmen had come in jeeps and surrounded the mosque before setting off the bomb.

This is one of the deadliest ever attacks in the region. Images on social media showed bloodied bodies covered with sheets.

President Abdel Fattah al-Sisi has pledged to respond with brutal force. “The armed forces and the police will avenge our martyrs and restore security and stability with the utmost force,” Sisi said in a televised address on Saturday. He has declared three days of national mourning.