Chief Justice Shirley Abrahamson wrote that the police actions violated the privacy rights of citizens in each instance.
"People do not buy cellphones to have them serve as government tracking devices," Chief Justice Shirley Abrahamson wrote in opposing the decisions. "They do not expect the government to track them by using location information the government gets from cellphones."
The court's rulings involved a pair of 2009 homicides. The case of Nicolas Subdiaz-Osorio, in Kenosha County, is the first known case in Wisconsin of police tracking a suspect's cellphone. The other case involved Bobby Tate, of Milwaukee.
Even though search warrants were not obtained in either case, the Supreme Court said in each circumstance police were within their rights to track the cellphones.
Abrahamson, who dissented in the Tate case along with Justice Ann Walsh Bradley, disagreed with the majority and said a search warrant was needed in both instances.
The Legislature earlier this year passed a law that prohibits law enforcement tracking cellphones without a warrant, but that was not in effect at the time of the cases that the court ruled on Thursday.
Department of Justice spokeswoman Dana Brueck said both cases were being reviewed and she had no immediate comment. John Pray, the attorney for Subdiaz-Osorio, said he was disappointed with the decision which he said further erodes privacy rights for cellphone users. Tate's attorney did not immediately return messages.
In the Tate case, witnesses told police they saw the shooter purchase a cellphone minutes before two people were shot, one fatally, right outside the cellphone store in Milwaukee. Based on that information, police got an order to trace Tate's cellphone and obtain location information such as cell tower activity.
Using that information, police tracked the phone to Tate's mother's apartment. They were given permission to enter the apartment, and found Tate asleep with the cellphone in his possession, along with a tennis shoe that appeared to have blood on it.
Tate was convicted after pleading no contest to first degree reckless homicide and possession of a firearm by a felon. Tate argued that police violated his 4th Amendment right against unreasonable searches and that there was no authority under the law to order that his cellphone be tracked.
The Supreme Court, in a 5-2 opinion written by Justice Pat Roggensack, agreed with lower courts and said the cellphone tracking was reasonable because it was done in accordance with a lawful court order.
A search warrant was not needed, Roggensack wrote, because the judge's order to track the cellphone complied with the spirit of the law requiring search warrants.
In the other case, Subdiaz-Osorio pleaded guilty to first-degree reckless homicide in the killing of his brother in Kenosha. Subdiaz-Osorio was arrested in Arkansas, after his cellphone provider turned over tracking information at the request of police.
Like Tate, Subdiaz-Osorio argued that his Fourth Amendment rights were violated, but Justice David Prosser wrote that circumstances of the case created an exception to the requirement to obtain a warrant.
Prosser, while describing the value of personal privacy and the ability to "enter and withdraw from public spaces and disclose the details of our thoughts and movements at our discretion," said a search warrant was not needed in part because of the delay it would have caused in tracking down the suspect.
Abrahamson filed the lone dissent. The five other justices agreed with Prosser, but came up with different reasons to support the conviction.