PARIS (AFP, REUTERS) - One of the two attackers of a church in northern France was obsessed with Syria and was radicalised very quickly, according to his family members and acquaintances.

Adel Kermiche, a 19-year-old French national, also struggled with psychological issues, said reports.

He and his accomplice stormed a Catholic church in the northern town of Saint-Etienne-du-Rouvray during morning mass on Tuesday (July 26) and slit the throat of an 86-year-old priest at the altar.

"It took two months (for him to become radicalised) and he was no longer my little brother. It was religion above everything… I don't know what happened to him, it was a real brainwashing," Kermiche's sister told local media.

His father also described him as a religious fanatic, the Guardian news website reported, citing French daily Le Monde.

Kermiche first came to the attention of anti-terror officials when a family member reported him as missing in March 2015.

He was arrested by German officials and found to be using his brother's identity in a bid to reach Syria. He was returned to France where he was detained on March 23 that year for "criminal association in connection with terrorism" and preparing a terrorist act.

He was released on bail but banned from leaving the Seine-Maritime region of northern France.

Six weeks later, Kermiche fled the family home once again and was ultimately traced to Turkey where he was detained on May 13, 2015. He was arrested on his return to France and remanded in custody before being released on bail subject to a curfew as he awaited trial for links to terrorism.

He was fitted with an electronic tag that allowed him to leave his house on weekdays between 8am and 12.30pm local time.

"We knew he wanted to go to Syria," said a 60-year-old neighbour, who added that he never saw Kermiche going to the mosque that the family attended.

"He never spoke to us," said the neighbour. "The last time I saw him was on Friday. He was playing football in his garden."

One of Kermiche's acquaintances from the area of Saint-Etienne-du-Rouvray told Le Parisien newspaper that he was a "time bomb" and a "hyperactive child" who was excluded from school at the age of 12 due to "behaviour issues".

"He only spoke about Syria, and his dream of killing (President) Bashar (al-Assad's) soldiers," they said.

Kermiche had struggled with psychological issues for much of his life, having been monitored from the age of six and hospitalised on several occasions in his teenage years - including 15 days in a specialist psychiatric unit, according to reports.

A teenager who said he knew Kermiche told RTL radio that he was not surprised by what happened.

"He talked about it all the time... He spoke about Islam, that he was going to do stuff like that. He told me two months ago 'I'm going to do a church.' I didn't believe him. He said a lot of things," said the teenager.

Another resident told RTL: "He was a young guy like us. I don't understand how he changed like that.

"What he did has nothing to do with Muslims, he did that of his own accord," he added.

Kermiche lived with his family in a modest home less than 2km from the church.

He and his accomplice, who is still unidentified, stormed the centuries-old stone church and took priest Jacques Hamel hostage, along with three nuns and two worshippers before slitting the elderly cleric's throat.

Sister Danielle, a nun who managed to escape, described how the attackers made the priest kneel at the altar in his white vestments.

"In the church everyone was shouting 'stop, you don't know what you're doing'. But they didn't stop. They forced him to get down on his knees. He tried to struggle, he tried," she told RMC radio.

"They recorded it. They did something like a sermon around the altar in Arabic," she added.

Sister Danielle then fled the church in a daring escape after which she was able to call the police. Three other hostages escaped unharmed while one worshipper had severe knife injuries to his throat.

Police shot dead both attackers outside the church after negotiations failed and they ran towards the police shouting "Allahu Akbar (God is greatest)", with one of them carrying a handgun.