International Mobile Subscriber Identity locators are under fire in this week’s hearing about the use of cellphone users’ phone data without their knowledge, reports.

This technology allows both the FBI and, in several documented cases, local law enforcement to filter through a large amount of cellphone data in a given region and locate one specific signal – and thus a suspect, the article says.

The trouble arises, privacy advocates say, when such tools (like the one in question, sold under the Stingray brand) operate by fooling cellphones into believing they are connecting to a cell tower when they are linking up[ to a surveillance tool. The technology also absorbs data of the potential suspect as well as all individuals within the given region.

So far, the U.S. government has argued that user privacy was upheld since authorities deleted all collected third-party data after the search.

But organizations such as the American Civil Liberties Union (ACLU) and the Electronic Frontier Foundation have issues with how the technology works – which they say violates the Fourth Amendment’s protection against “general warrants” – and the manner in which the FBI used it in several cases, including that of Daniel David Rigmaiden, a suspect in a tax fraud scheme.

However, both the FBI and the Department of Justice are basing their defense primarily on claims that the charges were an innocent mistake by agents “using a relatively new technology,” the article says. Secondly, the DOJ is expected to argue that Stingray technology does not encroach on a warrant-requiring search as cellphone use does not come with a reasonable expectation of privacy.