The US Department of Homeland Security has been purchasing and using large quantities of cell phone location data from data brokers, raising concerns about the government's ability to track individuals without a warrant. The data is collected from smartphone apps and can be used to identify patterns of life, track specific individuals, and learn details of private activities and associations. The ACLU has obtained thousands of pages of documents detailing the government's purchase and use of this data, and is advocating for Congress to pass the Fourth Amendment Is Not For Sale Act to prevent such practices. The government's actions have sparked debate about the balance between national security and individual privacy.