Soon his greatest wish was to travel to Syria to join the so-called Islamic State, which his mother said had "bewitched" him on social media.

On Tuesday, Kermiche was identified by prosecutors as one of the young men who slit the throat of a Catholic priest during mass in a church in northern France. He was shot dead by police with an accomplice after they entered the Saint-Etienne-du-Rouvray church near Rouen on Tuesday morning, cut the priest's throat and seriously injured another parishioner.

Kermiche was a local teenager, born in 1997, who had tried to join IS in Syria, Paris prosecutor Francois Molins said – though he failed in two attempts and instead served jail time in France.

His mother, a teacher, told the Tribune de Geneve in 2015 that the Charlie Hebdo attacks of January 2015 had "acted as a detonator" on a teen who had been cheerful, kind, sociable, music-loving and a regular mosque-goer.

"He said we could not practise our religion in peace in France," she said. "He was speaking with words that did not belong to him. He was bewitched, like he was in a cult."

His family found their son had secret Facebook accounts he used to talk to radicals around the world. After finding the Facebook accounts Kermiche's parents confronted their son but he argued back - and the next day he made a plan to go to Syria via Bulgaria and Turkey.

His first attempt ended in Munich, where he was caught, returned to France and put on probation, signing in once a week at the local police station and forbidden from leaving the country. His parents tried to reason with him but failed to change his views.

"I asked the authorities to put him in an electronic [tracking] bracelet but it was not possible," his mother said.

Six weeks later he disappeared again. He arrived in Geneva with another young runaway, AEM, and they asked around the airport for ways to get to Turkey – behaviour that was later reported to the authorities. Using his cousin's identity card, Kermiche flew to Istanbul.

Meanwhile in Paris anti-terrorism authorities had taken the case. The high court issued an arrest warrant and an alert went out to 28 countries. Kermiche was intercepted in Istanbul and put on a plane back to Geneva, where he was arrested in May 2015.

"Fortunately we managed to catch up in time – twice," his mother said. "If he had reached Syria I could have lost him. I want to know who messed up our child… we do not know what to do, we do not know where to turn for help."

Kermiche was extradited back to France from Geneva in May last year, indicted for conspiracy in connection with a terrorist enterprise and remanded in custody. But he was released from detention in March, Mr Molins said, and put under house arrest with an electronic bracelet.

Mr Molins said his office had appealed against this magistrate decision, "we wanted him to be in custody longer", but an appeal court confirmed the ruling. Kermiche was allowed to live at home and had the right to go out from 8.30am to 12.30pm during the week and 2pm to 6pm on the weekend.

The Tribune reported that Kermiche had also been involved in the radicalisation of another youth, AB, who left for Syria in 2015 when he was 16 using AK's identity card. The two lived in the same city and were friends on Facebook.

AB sent back to Kermiche information on how to get to Syria, including the phone number of a Turkish smuggler. AB's father said his son's radicalisation had been "meteoric" after the Charlie Hebdo attack.

L'Express reported that AB's younger brother, HB, was arrested on the street on Tuesday, under suspicion of having a connection with Kermiche. HB, 16, lived 200 metres from the Saint-Etienne-du-Rouvray church.

Mr Molins said a minor born in 1999 in Algeria had been remanded in custody.

Two relatives of one of the church attackers told RTL they were not surprised he had done it.

"He talked about it all the time… he talked about Islam… that he was going to do things like that, start with one of those. On the Koran he said to me 'I am going to attack a church'... On the life of my mother, I didn't believe him."

Another acquaintance told RTL the attacker was from the College Paul Eluard south of Rouen, and had been a young student "just like us".

"I don't understand why he changed," the acquaintance said. "I don't know why he fell into all that. His head got turned around and you mustn't compare him to other Muslims, that has nothing to do with it… It's not Muslims who want to do that, it's him, this individual. He was talking crap, that's all.

"He was a normal boy, a youth like anyone in the world. I don't know what took hold of him… his head got turned around. By who, when, where? No idea."

The fifth edition of the French version of IS's Dar al Islam magazine included an article calling for its followers to attack churches, Le Figaro reported.

"Always aim for well-populated areas such as tourist sites, supermarkets, synagogues, churches, Masonic Lodges, political party headquarters… the aim is to instil fear in their hearts," the newspaper quoted the magazine saying.

The Australian National Imams Council and The Grand Mufti of Australia released a statement to the media on Wednesday expressing shock and condemning Islamic State.

The Grand Mufti of Australia, Dr Ibrahim Abu Mohamed said "ISIS is an evil organisation that has hijacked the religion of Islam for its own brutal and nihilistic goals. The betrayal of the Islamic faith and violation of its core principles are enough to refute ISIS's false and absurd claims."