WASHINGTON—A federal probe revealed Friday that at least six U.S. border guards have been caught on tape attempting to boost arrests by planting illegal immigrants in cars entering the United States from Mexico. "In a typical scenario, suspected smugglers are asked to step out of their vehicles, and while one agent interrogates them, another slips an undocumented underneath the front seat," said investigator Douglas Jennings, showing grainy surveillance footage to reporters. "One immigrant we know of has been used as evidence in 12 separate cases." The report comes on the heels of allegations last week that Canadian border guards have been slipping extra medication into Americans' cars simply because the drivers looked like they could really use it.