In a rare defeat for law enforcement, the Supreme Court unanimously agreed on Monday to bar police from installing GPS technology to track suspects without first getting a judge's approval. The justices made clear it wouldn't be their final word on increasingly advanced high-tech surveillance of Americans.

Indicating they will be monitoring the growing use of such technology, five justices said they could see constitutional and privacy problems with police using many kinds of electronic surveillance for long-term tracking of citizens' movements without warrants.

While the justices differed on legal rationales, their unanimous outcome was an unusual setback for government and police agencies grown accustomed to being given leeway in investigations in post-Sept. 11 America, including by the Supreme Court. The views of at least the five justices raised the possibility of new hurdles down the road for police who want to use high-tech surveillance of suspects, including various types of GPS technology.