AT&T has reportedly been operating a secret surveillance programme, which law enforcement authorities in the US have used for a variety of investigations, without the use of a warrant. AT&T's secretive surveillance programme, called Project Hemisphere, allegedly involves locating a specific target and gathering his/her communication data, by searching trillions of call records and cellular data.

According to documents obtained by The Daily Beast, Project Hemisphere allows police to tap into AT&T's vast trove of metadata, which includes information such as location, and time and duration of phone calls, to expose key information about a target's activities. Project Hemisphere was first disclosed by the New York Times in 2013. The surveillance programme is described as a "partnership" between AT&T and the US government.

"This document here is striking," Adam Schwartz, staff attorney for EFF (Electronic Frontier Foundation) told The Daily Beast. "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 a minimum there is a very serious question whether they should be doing it without a warrant. A benefit to the parallel construction is they never have to face that crucible. Then the judge, the defendant, the general public, the media, and elected officials never know that AT&T and police across America funded by the White House are using the world's largest metadata database to surveil people."

However, according to the report, instead of a partnership, Hemisphere was designed as a specialised surveillance product, developed, marketed and sold by AT&T, at a cost of millions per year to taxpayers. Moreover, law enforcement agencies making use of Hemisphere reportedly required no warrant to access data. In return, police would agree not to disclose Hemisphere, in the event that an investigation using the surveillance programme became public.

The revelations come as the telecom giant seeks to acquire Time Warner, amid major concerns as US presidential candidates Hillary Clinton and Donald Trump have raised questions about the deal.

Documents reveal Hemisphere is currently being used in at least 28 intelligence centres across the US. AT&T employees reportedly perform data analysis on behalf of the police as it does not have direct access to the data.

Privacy groups such as the EFF, the American Civil Liberties Union (ACLU) and Electronic Privacy Information Center have all expressed concern over the use of Hemisphere, noting that such surveillance is unconstitutionally invasive.

An AT&T spokesperson said in a statement that the firm had "no special databse".

"Like other communications companies, if a government agency seeks customer call records through a subpoena, court order or other mandatory legal process, we are required by law to provide this non-content information, such as the phone numbers and the date and time of calls," AT&T's statement read.