Requiring law enforcement officers to get a warrant to use the location data created in the everyday operation of a cell tower would "cripple national security investigations before they reach their goal." That's the opinion that Jason Weinstein, the Deputy Assistant Attorney General of the Department of Justice, expressed to the Congressional Internet Caucus Advisory Committee last week. He also called cell tower data a "building block" of criminal investigations: where there's reasonable suspicion of guilt, agencies often rely on the data before turning to more intrusive methods such as GPS tracking once probable cause has been established.
The issue has come to light following the conclusion of US vs. Jones, which found that the use of a tracker on a suspect's car without a warrant violated his Fourth Amendment rights to privacy. However, US law also includes what's known as third party doctrine, where if you willingly supply information to a third party such as your bank or your cellphone provider, you lose your rights to privacy over it and the government can request it.
The subtlety between reasonable suspicion and probable cause is whether a warrant is required for the use of a given investigative technique. If an officer requires reasonable suspicion (or specific and articulate facts) to use or acquire data, no warrant is required. Under probable cause, they will need a warrant. It's down to courts (and ultimately congress) to distinguish the restrictions placed on law enforcement agencies on what they require to use each kind of data.
Weinstein's co-panellist Greg Nojeim, a Senior Counsel at the Center for Democracy and Technology, was unconvinced by the argument. He said that the claims that requiring a warrant would cripple law enforcement were exactly the same ones made at the US vs. Jones case, and that "not one Justice accepted the Department of Justice's arguments." He added, "Probable cause is not a straitjacket. It can be applied in ways that can both facilitate law enforcement investigations and protect privacy. It's up to congress to really make that happen."