A unanimous Supreme Court ruled on Monday that law enforcement needed a warrant when it placed a GPS tracking device to a suspected drug dealer's car.

"We hold that the government's installation of a GPS device on a target's vehicle, and its use of that device to monitor the vehicle's movements, constitutes a search," said Justice Antonin Scalia, writing for the majority of the court.

The case stemmed from the conviction of night club owner Antoine Jones on conspiracy to distribute five kilograms of cocaine and 50 or more grams of cocaine base. Law enforcement had used a variety of techniques to link him to the co-conspirators in the case including information gathered from a global positioning system that was placed on a Jeep primarily used by Jones. Law enforcement did not have a valid warrant to place the device on the car.

"It is important to be clear about what occurred in this case," Scalia wrote. "[T]he government physically occupied private property for the purpose of obtaining information. "

The decision is a loss for the Obama administration, which had argued that the attachment of the device to monitor the movements of Jones' vehicle on public streets was not a search.

"Today's decision means that the government does not get a free pass to do these sorts of activities,  because they need a warrant," said Hanni Fakhoury, staff attorney the Electronic Frontier Foundation,  a group that filed a brief supporting Jones.

"The decision is an important first step in combating the government's use of invasive and extensive GPS surveillance on individuals. The court went half way there and said that installing the device is unconstitutional without a warrant. We look forward to having the court address the second point which is the actual act of surveilling an individual. "

The court today affirmed a lower court ruling that reversed Jones' conviction because of the GPS evidence that was obtained without a warrant.

Scalia was joined by Chief Justice John Roberts and Justices Anthony Kennedy, Clarence Thomas and Sonia Sotomayor. Justice Samuel Alito - joined by Justices Ruth Bader Ginsburg, Stephen Breyer and Elena Kagan - agreed with the conclusion of the court but wrote separately because his legal reasoning differed from the majority.

Alito focused not on the attachment of the device, but the extent of time law enforcement monitored Jones.

"In this case, for four weeks, law enforcement agents tracked every movement that respondent made in the vehicle he was driving. We need not identify with precision the point at which the tracking of this vehicle became a search, for the line was surely crossed before the four-week mark," Alito wrote.