A judicial official said on Friday the probe concerns charges of forgery and alteration of documents.
The official said the decision to investigate was triggered by a complaint filed by civil parties in the 2016 case surrounding the murder of the priest in Saint Etienne du Rouvray by two extremists. The official wasn't authorized to speak publicly and couldn't be named.
The opening of a probe follows a report a day earlier by the online investigative publication Mediapart revealing the existence of an intelligence note on Adel Kermiche, alleging it was post-dated once the deadly attack on the 85-year-old Rev. Jacques Hamel took place and suggesting the murder might have been avoided.
The Mediapart article tackles the bureaucracy that allegedly kept the note about Kermiche from going up the command chain - and into the hands of the main intelligence service handling terrorism cases.
Based on months of interviews with police officers close to the intelligence operation, it cited low staffing on summer holidays, poor work conditions and, above all, the need for members of the hierarchy to sign off on intelligence documents submitted by the rank and file.
"Because what we write is classified as a defense secret, there are too many controls, too much rereading, too many chiefs who want to correct the notes, put their stamp on it ...," Mediapart quoted an unnamed officer in the intelligence unit as saying.
The Islamic State group claimed responsibility for the attack, as well as the attack in Nice earlier that month, July 14, when 84 people were killed by a man who plowed his truck down a seaside promenade.
The two 19-year-old attackers were killed by police as they left the St. Etienne church.
A statement by the Paris police chief's office, where the intelligence note originated, denied the Mediapart allegations, saying the note evoked neither an imminent act nor "the targeting of a precise place."
Once the attack occurred, the intelligence officer who wrote the note based on his intercepts on the encrypted Telegram channel "immediately made the link with the individual he had identified," the statement said. Then, "without delay" the Paris police intelligence arm informed investigators and wrote a new note, dated July 26, 2016 - the day of the attack. Police headquarters dated the initial note July 22, four days before the attack, while Mediapart date it July 21.
The original note, "which bore no urgent nature and was part of the service's daily detection work, followed the normal circuit of validation," the police chief's office said.
In contrast, Mediapart claimed that Kermiche, who had been arrested trying to go to Syria, said in a seven-minute-plus conversation on Telegram that because going to Syria is difficult it was best to carry out attacks on home soil, and cited churches as a target.