Abdel Malik Nabil Petitjean was identified by DNA tests as the other individual responsible for the stabbing of Fr Jacques Hamel near Rouen.
According to a police source speaking with the Reuters news agency, officers had opened a file on the 19-year-old in June for becoming radicalised.
Born in the eastern region of Vosges, Mr Jean and Adel Kermiche, also 19, entered the church in Saint-Etienne-du-Rouvray during morning mass and slit the 85-year-old's throat.
Describing the pair - who were later shot dead by police - as its "soldiers", the so-called Islamic State group claimed responsibility for the killing.
A number of people were taken hostage in the incident, including three nuns who were used as "human shields" by Mr Jean and Mr Kermiche when police entered the church.
An 86-year-old parishner was also seriously hurt.
The death of Fr Hamel has prompted fresh concerns over the safety of church-goers in the UK, while the National Police Chiefs' Council (NPCC) urged churches to review their security as a precaution.
In a statement released yesterday, the NPCC added: "While there is no specific intelligence relating at attacks against the Christian community in the UK, police are urging the community to be alert but not alarmed..."
Mr Kermiche had been released from prison after twice trying to use the identities of relatives to join extremists in Syria and had been wearing an electronic surveillance tag at the time of the attack.
According to anti-terrorism prosecutor Francois Molins, his tag was switched off for a time every morning, and the murder took place while it was not functioning.
Premier has been unable to confirm the two men pictured above are Abdel Malik Nabil Petitjean and Adel Kermiche.