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In many cases, the issue of whether or not there has been an impermissible acquisition of evidence will involve two different questions: (1) was there an illegal seizure of the suspect (detention or arrest) leading to discovery of the evidence, and (2) was there an unlawful search of the suspect or his property during which the evidence was found?
In the usual case, both the seizure and the search must be reasonable under the Fourth Amendment in order for the evidence to be admissible. The U.S. Supreme Court and federal appeals courts have considered both issues when officers have used K-9s to detect contraband.
The "Seizure" Issue
If you make an unjustified detention or arrest in order to get an opportunity to search, any evidence you then find will generally be "tainted" by the fact that it resulted from an unreasonable seizure. (Florida v. Royer) In most cases, a detention must be based on reasonable suspicion that the individual is involved in criminal activity (Terry v. Ohio).
An exception to this rule allows suspicionless stops at certain checkpoints that the court has said do not violate the Fourth Amendment. To date, these have been limited to vehicular checkpoints to make brief inspections for immigration compliance (U.S. v. Martinez-Fuerte), driver's license and registration (Delaware v. Prouse), driving under the influence (Michigan v. Sitz), and witnesses to recent serious crimes. (Illinois v. Lidster) Based on these exceptions, the City of Indianapolis decided to establish vehicular checkpoints to check for narcotics.
Under the Indianapolis procedure, police set up lighted roadway signs warning motorists "Narcotics Checkpoint 1 Mile Ahead. Narcotics K-9 in Use. Be Prepared To Stop." At the checkpoint, a predetermined number of vehicles would be detained only long enough to run a dog around the car and ask a few questions, while other traffic proceeded without interruption. The "hit" rate on the stopped vehicles was a fairly high 9 percent.
Two motorists who had been stopped at the drug checkpoint sued the City of Indianapolis and its police department, claiming a violation of their Fourth Amendment right against unreasonable seizure. The Supreme Court agreed that the checkpoint violated the Fourth Amendment and allowed the suit to proceed.
The court pointed out that other kinds of checkpoints it had approved had some primary purpose other than detection of criminal activity (border control, highway safety, or locating witnesses). None of them had the essential purpose of crime detection. This distinction made the checkpoint detentions in Indianapolis different, and unreasonable.
The court said this:
"It is well established that a vehicle stop at a highway checkpoint effectuates a seizure within the meaning of the Fourth Amendment. What principally distinguishes the Indianapolis drug-detection checkpoints from those we have previously approved is their primary purpose-interdicting illegal narcotics.
"We have never approved a checkpoint program whose primary purpose was to detect evidence of ordinary criminal wrongdoing. Rather, our checkpoint cases have recognized only limited exceptions to the general rule that a seizure must be accompanied by some measure of individualized suspicion. Because the primary purpose of the Indianapolis narcotics checkpoint program is to uncover evidence of ordinary criminal wrongdoing, the program contravenes the Fourth Amendment" (Indianapolis v. Edmond).