Egypt's President, Abdel Fattah el-Sisi identified the bomber as 22-year-old Mahmoud Shafiq Mohammed Mustafa, who had worn a suicide vest.

[12] An unnamed church source told a Nile TV reporter that the bomb was thrown inside the cathedral's hall, adjacent to the entrance of the building.

While security officials did not know the bomber's gender,[11] some media organizations speculated that the assailant was a woman since most victims were women and children.

Prime Minister Sherif Ismail and Cairo's security chief, General Khaled Abdel-Aal, visited the scene.

[19] However, an Interior Ministry official, police Maj. Gen. Tarek Attia, said that the suicide bomber had been arrested in 2014 in Fayoum province, southwest of Cairo, on charges of being a member of the outlawed Muslim Brotherhood.

[18] Mahmoud Shafiq Mohamed Mostafa, the man identified by Egyptian authorities as the perpetrator of the St. Peter and St. Paul church bombing was reportedly arrested in 2014 in Hawatem Square in Fayoum along with a colleague, according to lawyer Yasmine Hossam al-Din, who represented him at the time.

A year after his arrest, in May 2015, the prosecution dropped charges of arms possession and joining a banned organization, but upheld accusations of protesting without a permit, meaning the case was categorized as a misdemeanor rather than a felony.

Fayoum Criminal Court, headed by Atef Rizk, ordered his release in 2015, after Mostafa exceeded the maximum 6-month pretrial detention period for misdemeanors.

Security forces later arrested Mostafa’s colleague, according to Hossam al-Din, and detained him pending investigations into another case in Beni Suef..[20] The Egyptian Interior Ministry produced a statement asserting that the Cairo attack was organized and carried out by terrorists led by Mohab Mostafa El-Sayed Kassem, which the Ministry said had links to Ansar Bait al-Maqdis, also known as Islamic State of Iraq and the Levant – Sinai Province.

[26] Bishop Youssef of the Coptic Orthodox Diocese of the Southern United States issued a press release saying the church would be fasting and praying "not for the martyrs" but for the healing of the Egyptian people and for Muslims and Christians to live peacefully together.