Pippa Middleton was facing a grilling by outraged cops in Paris after she was seen in a car with friends as one brandished a gun.
The stunt came as France reels from a series of gun massacres across the country.
Prince William's sister-in-law was with three male pals in an Audi convertible when the driver took out what appeared to be a weapon and held it aloft in broad daylight.
He waved it in the air and pointed it at a photographer, who captured the chilling moment the barrel of what looks like a semiautomatic gun stared straight down his lens.
Middleton, the 27-year-old sister of the future British queen Catherine, 30, is clearly seen in the front passenger seat.
The Paris Judicial Police were poised to launch a probe, as a source said Middleton faces arrest amid a gun clampdown.
"If the evidence points to her involvement, she will be prosecuted," the source said. "Anybody involved in the illegal use of a handgun in public is liable to arrest and interrogation."
The source confirmed that if the gun was real, brandishing it in a public place is punishable by up to seven years' jail "for all parties involved." Even if it was fake, the maximum jail term is two years.
"Certainly, if you were in a car and knew somebody was using a gun to warn people off, you'd be liable to arrest," the source said.
The French photographer was "considering his options" over making a formal police complaint Sunday night.
A security source said, "Waving a weapon in public is not only illegal -- it's incredibly stupid."
Middleton was understood to have been in Paris for a weekend party.