Justin Bieber -- Probation Violation Investigation Launched After Arrest

Justin Bieber is in the crosshairs of the L.A. County Probation Dept ... because his Canadian arrest just set in motion a chain reaction that could land him behind bars.

Law enforcement sources tell us an investigation has been launched to determine if Justin's arrest for dangerous driving and assault constitutes a violation. We're told officials will contact law enforcement in Canada to verify the details of the incident.

Justin is currently on 2 years probation after pleading to misdemeanor vandalism in the egging case back in July.

Cops in Canada say Bieber got into a physical altercation with a paparazzo after his ATV collided with the photog's minivan.