Anti-terrorism unit investigators found that Adel Kermiche and Abdel-Malik Petitjean, both 19, started communicating on the app on July 22 -- just four days before they fatally stabbed Rev. Jacques Hamel, 86, in the church in Saint-Etienne-du-Rouvray in northern France, the source said.
They used the app to coordinate their plans for the attack, the source said.
"Someone put them in touch. Investigators are still trying to figure out who this person was," the source told CNN.
Telegram allows users to send messages protected by end-to-end encryption -- the kind authorities can't intercept easily.
Released three years ago, the app has become known as a preferred means of communication for the Sunni terror group ISIS, and was used by the ISIS cell that plotted the Paris terror attacks in November.
Kermiche and Petitjean, who were shot dead by police as they left the church in the wake of the killing, carried out the attack in the name of ISIS. The terror group's media wing subsequently posted a video on Telegram showing the pair pledging allegiance.
Attacker's cousin charged
The attack in the quiet town during a morning sermon -- during which the priest's throat was slit at his altar and four other people taken hostage -- was just the latest in a wave of terror attacks that have fueled French citizens' concerns over their government's ability to respond to the domestic terror threat.
Kermiche was known to authorities as a radicalized individual and even wore an electronic monitoring device during the fatal hostage incident, according to French prosecutor Francois Molins. He had previously traveled abroad to try to fight in Syria.
The church attack came less than two weeks after the Bastille Day terror attack by an ISIS supporter that left 84 people dead in Nice.
On Sunday, the Paris anti-terrorism prosecutor's office said that the cousin of one of the attackers has been arrested and charged
in connection with the attack.
Farid K, a 30-year-old cousin of Petitjean, was aware "of an imminent violent attack planned by his cousin," the prosecutor's office said in a statement.
Muslims pay respects in Catholic churches
On Saturday, a vigil was held for Rev. Hamel in the church in Saint-Etienne-du-Rouvray.
And on Sunday, Muslims attended masses in churches around France to pay their respects to the slain priest.
Muslims also attended Catholic masses in cities across Italy, in a show of solidarity at a time of heightened religious tensions across Europe.
French Interior Minister Bernard Cazeneuve told a press conference Monday that 20 mosques across the country had been closed since December for radical preaching, saying that "there will be others."
"There isn't a place in France for those who incite hatred in prayer rooms and mosques," he said.
France has been in a state of emergency since 130 people were killed in the Paris terror attacks in November.
Muslim community leaders in the Normandy town where the priest was killed have refused to bury Kermiche
, according to the president of a local Muslim organization. The mayor's office will make the final decision on whether Kermiche can be buried in the town.