In a 6-3 decision, the US Supreme Court held that reasonable suspicion is needed before an officer may involuntarily hold a driver pending a dog sniff of the vehicle.   In this case, the driver was stopped for briefly driving on the shoulder of the road.  The officer completed the infraction and asked the driver if he could walk his dog around the perimeter of the car.  The driver refused, but the officer decided to proceed anyway.  The officer called for backup and held the driver for an additional 7-8 minutes awaiting backup.  Once the backup officer arrived, the officer conducted the dog-sniff.  The dog alerted and meth was found.     

The Court held that absent reasonable suspicion, the officer could not expand the scope of the investigation and could not lawfully detain the driver beyond that necessary for the lawful basis of the stop.     The dissent argued the court should not have reached the legal issue because the officer actually had reasonable suspicion in the case, and the decision is inconsistent with prior decisions.  The dissent may ultimately prevail on this point, it was remanded for the 8th Circuit to address that issue.

Unresolved in this case is the extent to which officers are permitted to investigate beyond the infraction at hand when the detention doesn't involve a dog-sniff.  Many officers, as in this case, pose questions intended to uncover criminal behavior.  If these questions are peppered throughout the detention and extend the length of the stop, is it a 4th amendment violation or is it "reasonable" in light of the overall circumstances?  Justice Alito, in a footnote to his dissent writes, "it remains true that [during the stop] police may ask questions aimed at uncovering other criminal conduct.  But it is not clear the majority would agree; saying only that an officer "may conduct certain unrelated checks" during a stop, without discussing what that implies.  

The full case is here: