Washington (CNN) -- Police erred by not obtaining an extended search warrant before attaching a tracking device to a drug suspect's car, the Supreme Court said in a unanimous ruling Monday.

A majority of justices said that secretly placing the device and monitoring the man's movements for several weeks constituted a government "search," and therefore, the man's constitutional rights were violated.

Four other justices also concluded that the search was improper but said it was because the monthlong monitoring violated the suspect's expectation of privacy.

That difference of legal analysis may create further confusion among law enforcement over when and for how long such high-tech operations can be used, on both criminal suspects and the general public.

At issue was whether movement in a private vehicle on city streets is "public" in nature.