The US government is pushing to be allowed access to people's cellphone records to help fight crime.
The Justice Department on Friday asked an appeals court in Philadephia to overturn previous rulings barring it from asking service providers about call activity to and from phones believed to belong to criminals.
It wants access to dates, times, call duration and location of callers - meaning, presumably, that providers would have to start keeping all this information indefinitely.
Lawyers for the Electronic Frontier Foundation and the Center for Democracy and Technology argued that this was an invasion of privacy. They said that the government should have to get a search warrant and show probable cause that the information would provide evidence of a crime.
But Justice Department lawyers argued that there was no consitutional difficulty about the proposal, saying the government was legally free to use tracking devices without obtaining a warrant.
The court will make its ruling later this year.