As Congress eyes the impending AT&T/Time Warner mega-merger, a disturbing new report reveals AT&T secretly spies on its own customers using a covert program called Project Hemisphere, at taxpayers’ expense.

In 2013, the New York Times reported on a “partnership” between the U.S. government and AT&T to help intelligence gathering in the war on drugs. But as the Daily Beast reported Tuesday, the massive telecommunications company secretly uses Project Hemisphere to assist law enforcement in gathering myriad information on other fronts, from Medicaid fraud to homicide investigations.

As the Daily Beast notes, law enforcement is not required to obtain a warrant before tapping into Project Hemisphere’s “massive trove of data”; in fact, law enforcement is barred from disclosing use of the covert project in the event that an investigation using it becomes public.

ACLU technology policy analyst Christopher Soghoian explains that while companies are required to provide data to law enforcement upon request, “AT&T doesn’t have to data-mine its database to help police come up with new numbers to investigate.”

In a 2014 statement, AT&T downplayed the impact of Project Hemisphere, insisting law enforcement “agrees not to use the data as evidence in any judicial or administrative proceedings unless there is no other available and admissible probative evidence.”

But Adam Schwartz, staff attorney for activist group Electronic Frontier Foundation, told the Daily Beast AT&T’s statement is problematic, because those charged with a crime have a right to hear the evidence against them. This means that in the event information leading to an arrest was gathered through Project Hemisphere, investigators may hide the origin of the lead by constructing a false narrative to provide the same evidence, through a dubious process called “parallel construction.”

“This document here is striking,” Schwartz said. “I’ve seen documents produced by the government regarding Hemisphere, but this is the first time I’ve seen an AT&T document which requires parallel construction in a service to government. It’s very troubling and not the way law enforcement should work in this country.”

AT&T on Saturday reached a deal to purchase Time Warner for $85 billion. The proposed merger drew immediate scrutiny from both sides of the aisle, including presidential candidates Donald Trump and Hillary Clinton. The consensus is that the deal would be disastrous for consumers. Federal antitrust regulators must approve the deal before the merger can move forward.