The man was identified later as Sid Ahmed Ghlam, French Interior Minister Bernard Cazeneuve told television broadcaster TF1.
Paris Prosecutor Francois Molins said Ghlam asked for medical help at his home in Paris' 13th district Sunday morning, claiming he had accidentally injured himself when he mishandled a weapon.
But authorities found more than a man with a gunshot wound in his thigh. Molins said Ghlam was someone suspected of killing a 32-year-old woman hours earlier and who then intended to launch a larger-scale terror attack in the near future.
"(He) let them believe that he was the victim," French President Francois Hollande said Wednesday. "But police understood that there was an investigation that needed to be initiated."
That probe started in earnest when police spotted traces of blood and bags in the suspect's car.
Based on this discovery "and in view of the attitude and suspicious behavior of the individual," they looked inside the car and found a loaded Kalashnikov gun, a 9-mm revolver, three cell phones, a laptop, a USB key, a satellite navigation system "and handwritten documents that contained information on possible targets," Molins said.
A search inside his home turned up yet more potentially key evidence, including three more Kalashnikovs, three bulletproof vests, police armbands, a camera, some 2,000 euros in cash and "documents in Arabic mentioning al Qaeda and ISIS," according to the prosecutor.
"A terrorist attack has been foiled," Hollande said. "It's not the first time."
Prosecutor: Algerian native was in contact with someone in Syria
Earlier Ghlam was identified as a computer science student originally from Algeria.
According to Molins, he lived in that North African country until 2001, joined his parents in France for the next two years, then went back to Algeria. He got his undergraduate degree in that nation, then returned to France to study electronics.
He'd never been convicted of a crime; in fact, the lone accusation against him in August 2013 was later withdrawn.
But that didn't mean French authorities didn't know about Ghlam.
"He had been noticed wishing to go to Syria," Molins explained. The prosecutor added that authorities found nothing to suggest he was imminent threat, "but he was under surveillance."
Law enforcement checked on the man once in 2014 and again this year, Cazeneuve said. The minister said the man planned to head to Syria, a country embroiled in a bloody, years-long civil war that has attracted foreigners wishing to join extremist militant groups like ISIS.
There was no indication he ever made it. But, Molins said, computer searches revealed that the man had been communicating with someone in Syria who "asked him to target a church."
Slain woman called Paris' first terror victim since January
What church? Authorities didn't disclose that detail on Wednesday, though Molins did say "this target was confirmed by (the suspect's satellite navigation system) and handwritten documents in his car."
Paris is home to the famed Notre Dame cathedral and Sacre Coeur basilica, both huge draws for tourists. It's not known if either were in line to be attacked, but Prime Minister Manuel Valls Wednesday visited one of the churches allegedly targeted, which was located in the Paris suburb of Villejuif.
Assuming Ghlam doesn't walk free anytime soon -- for now, he's in custody awaiting possible charges -- that attack won't happen anytime soon.
But authorities say he's already claimed at least one victim.
Her name is Aurelie Chatelain. The 32-year-old was found dead Sunday morning sitting in the passenger seat of a parked car in Villejuif, killed by a single bullet that struck her shoulder, went through her body and ended up embedded in the seat.
Cazeneuve said Chatelain was the mother of a 5-year-old girl and had come to the region on Saturday to attend a training class.
It's not known what connection, if any, she had with the suspect. Nonetheless, Molins said that ballistics, DNA, satellite navigation and other evidence connected him to the death of Chatelain, who Molins called the region's first victim of terrorism since January.
That's when the massacre at Charlie Hebdo's offices and the siege of a kosher market
in Paris took place, prompting French authorities to beef up security measures.
"France, like other countries, is facing a terror threat that is unprecedented in its nature and magnitude," Valls said. "... Terrorists are targeting France to divide us."