It held that use of an electronic beeper device to monitor a can of ether without a warrant constituted an unlawful search.

However, the Court upheld the conviction of Karo and his accomplices, stating that the warrant affidavit contained enough information not derived from the unlawful use of the beeper to provide sufficient basis for probable cause.

[1] Drug Enforcement Administration agents installed an electronic beeper in a can of ether with the consent of the owner, a government informant.

In United States v. Knotts,[2] the Court held that the monitoring of a beeper did not violate the Fourth Amendment when it revealed no information that could not have been obtained through visual surveillance.

The Supreme Court held that the use of the beeper to conduct surveillance on Karo and his accomplices constituted an unlawful search and seizure in violation of the Fourth Amendment.