If independently confirmed, Mahmoud Shafiq Mohammed Mustafa would be the latest Egyptian to be radicalized after being subjected to police abuse, a practice that was common for decades and has become rampant after an crackdown on dissent following the military's 2013 ouster of an Islamist president.
Speaking after a state funeral for the victims, Egyptian President Abdel-Fattah el-Sissi said the suspect detonated a belt of explosives inside a chapel adjacent to St. Mark's Cathedral, seat of Egypt's ancient Coptic Orthodox Church. The over 100-year-old chapel was packed with worshippers.
The dead included more than 20 women and children. Forty-nine others were injured, according to the latest figures released by the Health Ministry.
Mahmoud Hassan, one of Musafa's lawyers, said his client, who was 16 at the time of his arrest, was tortured until he confessed to the possession of weapons and explosions. He also faced charges of membership in an "illegal organization," Egyptian parlance for the outlawed Muslim Brotherhood, the Islamist group of which former President Mohammed Morsi was a senior official.
Musafa was released on bail after nearly two months in detention, and a court later convicted him in absentia, according to the lawyer. Traumatized by the torture, he told his lawyer not to appeal, fearing he would be abused again if detained.
A police photo of Mustafa and a friend arrested on the same day showed the pair, clearly in their teens, with bleeding noses and bruised faces. Placed atop a coffee table in front of them was a rifle, ammunition and what appeared to be a homemade bomb.
Hassan insisted that Mustafa was not a member of the Brotherhood, but the young Egyptian student from the oasis province of Fayoum appears to have been radicalized by his experience in detention, a danger many Egyptian rights activists warn the government against. The activists seek freedom for anyone not involved in acts of violence.
Two local news websites on Monday quoted Mustafa's mother as saying he had not been home in two years.
No one has so far claimed responsibility for Sunday's bombing. Two active militant groups believed to have links to the Muslim Brotherhood group — Hasm and Liwa el-Thawra — distanced themselves from the attack. The local affiliate of the Islamic State group has so far remained silent.
Sunday's bombing was among the deadliest attacks in recent memory to target Egypt's Coptic minority, which makes up around 10 percent of the country's population and strongly supported the el-Sissi-led military overthrow of Morsi, Egypt's first freely elected president whose one year in office proved to be divisive.
Since then, Islamic militants have carried out scores of attacks, mainly targeting the security forces, while the government has waged a wide-scale crackdown on dissent.
Mustafa raised suspicions when, according to footage from a security camera, he hurriedly entered the chapel wearing a bulgy jacket, prompting one security guard to follow him. About 10 seconds later, Mustafa blew himself up.
The guard who followed him was killed, said Khaled Diaa-Eldeen, who leads a team of prosecutors investigating Sunday's attack.
Three men and a woman were arrested in connection with Sunday's attack and other suspects were on the run, el-Sissi said.
The president did not link the bomber to any militant groups, but a top Interior Ministry official appeared to confirm a key part of the account given by Mustafa's lawyer. Police Maj. Gen. Tarek Attia told The Associated Press the suspected bomber was arrested in Fayoum in 2014 and charged with membership in the Brotherhood.
"This strike really hurt us and caused us much pain, but it will not break us," el-Sissi said.
He also called on the government and parliament to introduce legislation that would allow more "decisive" methods of dealing with militants. He did not elaborate. Lawmakers and pro-government media have been promoting the referral of terrorism-related cases to military tribunals, which are notorious for their swift and harsh sentences.
During the funeral, coffins were wrapped in Egyptian flags and carried by military policemen. The service was attended by top government and military officials and guarded by hundreds of soldiers. Earlier on Monday, the Coptic community held its own funeral service at a suburban church.
"God, protect us and your people from the conspiracies of the evil ones," Pope Tawadros II, spiritual leader of Egypt's Orthodox Christians, prayed after waving incense over coffins lined up in front of the altar along with crosses made of white roses. "It is the destiny of our church to offer martyrs."
Only victims' relatives were allowed to attend the service at the Virgin Mary and St. Athanasius church in the eastern Cairo suburb of Nasr City.
Outside the church, a crowd of several hundred protesters scuffled with security forces when they were barred from attending the service.
"People have a volcano of anger inside their chests," said Nora Sedki, a Christian government employee who joined the demonstration.
"The blood of our brothers is dear," chanted the protesters, who carried Egyptian flags and crosses made of tree branches.
Soon after Morsi's 2013 ouster, his supporters lashed out at Christians, ransacking and destroying scores of churches and Christian-owned properties in southern Egypt, where sectarian tensions are more pronounced.
Successive Egyptian governments insist that Christians are not discriminated against, but many claim they have been denied top jobs, including in academia and in security and diplomatic services. They say it is the result of deeply rooted bias by the country's Muslim majority.
Christians have also accused the security forces of failing to do enough to protect them from Muslim extremists, a complaint that has persisted under el-Sissi's rule.
"The pope must take stronger action," said Amal Saad, one of the protesters, whose aunt was among the victims. "We can't keep weeping and burying the dead then nothing happens."
Associated Press Writer Mariam Fam contributed to this report.
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