Fr Jacques Hamel was killed by two men armed with knives in a church in France yesterday
Petitjean was identified through an ID found at Kermiche's home, French media reported, and he is believed to have been on a watch list like Kermiche.
DNA tests are being carried out on the second attacker to confirm his identity, judicial sources told Reuters.
Security forces in France are now under scrutiny after it emerged Kermiche was under house arrest and wearing a tag, having twice tried to travel to Syria.
His tag was turned off for a few hours each morning to allow him to leave home - and it was in this time that Kermiche and Petitjean slit the 86-year-old's throat.
A third person, believed to be a 17-year-old male, is in custody in connection with the attack.
Petitjean was identified as churches across France held memorial services for the priest.
Interior Minister Bernard Cazeneuve said France would bolster the operational reserve of its police force after 2,500 people asked to join up in the days after 84 people died in an IS-inspired attack on Bastille Day in Nice.
He also said more of the country's 10,000-strong Operation Sentinel anti-terror forces would be deployed to areas outside Paris following the Nice attack and the killing in Normandy.