ISIS has claimed responsibility for the attack on a church in Cairo on Sunday that killed 25 people.
In a statement by its news agency Amaq, the militant group said the suicide bomber, who it identified as Abu Abdallah al-Masri, had detonated his explosive belt inside the church.
"Every infidel and apostate in Egypt and everywhere should know that our war... continues," it said.
The attacker's lawyer, meanwhile, said he was radicalised after he was tortured by police aged 16.
Egpytian authorities identified Mahmoud Shafik Mostafa, who they say was 22, as the bomber. Mostafa's lawyer, Mahmoud Hassan, said he was arrested and abused by police after taking part in an Islamist demonstration two years ago, leaving a discrepency in his age.
Hassan said Mostafa was detained for two months in 2014 and tortured until he confessed to possessing weapons and explosives. He was then charged with membership of an "illegal organisation", in reference to the banned Muslim Brotherhood.
He said his client had been radicalised by his experience in jail.
In an interview with Reuters, Mostafa's mother said he had been sexually abused in police custody in 2014, but that she had seen no sign that he had been radicalised.
Egyptian President Abdel Fattah al-Sisi took power in 2013, deposing the Brotherhood's Mohamed Morsi, and has since outlawed the Islamist movement as part of a crackdown in which hundreds of its supporters have been killed and thousands jailed.
The Interior Ministry said Mostafa had been arrested in March 2014 for carrying arms during a protest, and freed on bail after two months. It said he had joined a cell led by Mohab Mostafa Sayyed Qassem, a militant with links to Islamic State fighters in Northern Sinai and exiled Brotherhood officials in Qatar, and was wanted in two other cases.
His mother, Umm Bilal, said Mostafa had fled to Sudan shortly after being released.
"Mahmoud would not do this... he would not kill anyone," she shouted as she listened to a news report on the bombing.
She said his father had died two years ago, and that Mostafa called her regularly from abroad, most recently about a week ago.
"He said he would not return because security forces would detain him again... He was asking after me and his sisters... I didn't notice any change in his voice or anything to suggest he would blow himself up."
"He kept crying all night... He wasn't crying because he was beaten or tortured, though the scars were still visible on his face and body," she said. "I believe they broke him at the station, they violated his honor."
But she said her son had not been radicalised.
She said his two brothers had also been arrested, one of them after the bombing.
CCTV footage released after the bombing shows a figure calmly walking into the Saint Peter and Saint Paul Church, a chapel adjacent to Cairo's St Mark's Cathedral, before the building explodes moments later.
The Muslim Brotherhood won Egypt's first free elections after the 2011 uprising that ended Hosni Mubarak's 30-year rule. Morsi became president, but was toppled two years later after mass protests.
Egypt's oldest Islamist organisation says it is peaceful, but has split into rival wings since the crackdown, while some supporters have formed splinter groups that have carried out attacks on police and judicial officials.
Disillusioned by its ill-fated flirtation with democracy, some younger Brotherhood supporters have gone to fight in Syria or joined the local arm of Islamic State, Sinai Province.
Since 2013, the group has killed hundreds of soldiers and police in the Sinai Peninsula, and it has started to attack Western targets within Egypt. The army has responded with air strikes and by bulldozing entire villages.
Additional reporting by Reuters.
