David Chazan in Paris reports:

One of the terrorists has been identified as Adel K., 19, known to have been radicalised and on a watch list as a potential threat to national security.

Police have searched his parents’ home only 1.5 miles from the church.

They used a bomb disposal robot to check for explosives or traps before they entered.

He started making contact with radicals on the internet after the Charlie Hebdo and kosher supermarket attacks in January last year.

He came to the authorities’ attention when he tried to help an underage teenager from Saint-Etienne-du-Rouvray join Isil.

He twice attempted to go to Syria but was arrested once in Munich and then turned back by the Turkish authorities and sent to Geneva, in May 2015.

The Swiss authorities charged him with “criminal association in connection with terrorism”. He was sent back to France and jailed for ten months.

He was tagged when he came out of prison in March this year. The public prosecutor appealed unsuccessfully against his release.

A French security source said he was in contact with Maxime Hauchard, a French jihadist identified as an Isil executioner. Hauchard, who come from the region. converted to Islam and frequented the mosque in Saint-Etienne-du-Touvray, appeared in a video showing the beaheding of American Peter Kassig.

The mother of Adel K. told a Swiss newspaper in May last year that his transformation into a radical happened very rapidly.

Previously he was a “happy kid who liked music and going out with girls”. But he quickly became a recluse, only going out to the mosque.

“He said you can’t practise your religion undisturbed in France. He spoke with words that were not his own. He was under a spell, like in a sect" said his mother, a teacher. His family and his brothers and sisters tried to reason with him and keep an eye on him.

Security sources said his Facebook posts show how he changed as he made little effort to hide his new sympathies.