Here's the story. Justin was in Ottawa, Canada in November when a female employee of the venue where he was performing filed a police report, claiming Bieber -- or someone who was Biebs adjacent -- assaulted her.
The woman told Canadian cops she was hit by something.
We now know what the something was ... a kid-safe projectile from a Nerf gun. Sources familiar with the situation tell TMZ ... Justin was Nerf fighting with his 3-year-old brother and 4-year-old sister and apparently one of the darts hit the woman.
We're told the woman pitched a fit and told Bieber's people she was a security guard for the venue and was royally pissed off. She filed a police report and the cops are actually taking it seriously -- they've already interviewed some people at the venue.
Getting hit by a Nerf projectile is definitely annoying, but criminal? Be serious, eh!