"The Justice Department’s unfortunate decision leaves Americans with no clear understanding of when we will be subjected to tracking — possibly for months at a time — or whether the government will first get a warrant," Catherine Crump, an attorney for the ACLU, wrote in a blog post.
"Privacy law needs to keep up with technology, but how can that happen if the government won’t even tell us what its policies are?" she added.
Crump said the ACLU will ask the courts to force the Justice Department to release the information.
The ACLU first learned of the existence of the memos when FBI General Counsel Andrew Weissmann mentioned them during a panel discussion at the University of San Francisco Law School last February.
He said the memos provide guidance to agents in light of the Supreme Court's decision in United States v. Jones, which held that installing a GPS device on a suspect's car qualifies as a search under the Fourth Amendment.
The first memo offers instructions on whether the decision applies to vehicles other than cars, like boats or planes, he said. The second memo guides agents on whether the decision restricts their ability to use other location-tracking tools.