French officials have identified the second man involved in the murder of a priest in a Normandy church during morning Mass.

The French prosecutor's office said he was Abdel-Malik Nabil Petitjean, 19, from eastern France.

He was spotted last month in Turkey as he supposedly headed to Syria but returned to France instead.

The prosecutor's office identified him following DNA tests on his corpse.

A security official confirmed that he was the unidentified man pictured on a photo distributed to French police on July 22 with a warning that he could be planning an attack.

Four days later, Petitjean and another 19-year-old local man, Adel Kermiche, stormed the church in Saint-Etienne-du-Rouvray during Mass on Tuesday.

They held five people hostage - the priest, two nuns and an elderly couple - before fatally slashing the priest's throat and seriously wounding the other man.

Another nun at the Mass slipped away, raised the alarm, and the attackers were killed by police as they left the church.

The attack was claimed by the Islamic State group, which released a video allegedly showing Kermiche and his accomplice clasping hands and pledging allegiance to the group.

Petitjean was born in eastern France, in Saint-Die-des-Vosges, but recently lived in the Alpine town of Aix-les-Bains where his mother lives, the prosecutor's office said.

Kermiche was from Saint-Etienne-du-Rouvray, where the attack took place in north-west France.

A youth believed to be 16 was detained after the church attack and is still being held for questioning.

Turkey spotted Petitjean at a Turkish airport going to Syria on June 10, said a security official.

On June 29 he was flagged to French authorities and immediately put on a special watch list.

Petitjean did not go to Syria but turned around and returned to France on June 11, said the official.

It is not clear what caused Petitjean to turn around.

However in recent months IS propaganda has encouraged Western recruits in particular not to join extremists in the war zones in Syria or Iraq but to remain home and carry out attacks.

The French anti-terrorism coordinating agency Uclat issued the photo of a man on July 22.

It warned police that the person - without a name but who turned out to be Petitjean - "could be ready to participate in an attack on national territory."

Uclat said the person in the photo could already be present in France and act alone or with other individuals.

It was not immediately clear how the two men knew each other or when Petitjean travelled from eastern France to Normandy in the west.

The church attack came less than two weeks after a man barrelled his truck down a pedestrian zone in Nice, on the Riviera, that killed 84 people celebrating France's national day, Bastille Day.

IS also claimed responsibility for that attack as well as two others that followed in Germany.

A gathering this weekend to honour victims of the Nice attack was cancelled on Thursday after authorities said law enforcement was too busy protecting against threats.

However a march on Thursday in Saint-Etienne-du-Rouvray is still expected to take place.