A civil liberties group has discovered emails showing that the US government has concealed police use of fake mobile base stations which can spy on citizens without requiring search warrants since at least 2009.

Emails obtained by the American Civil Liberties Union show that federal law enforcement officials insisted police in Sacramento, Florida conceal their use of the devices, known as "StingRay", in order to prevent criminals from learning how to get around them.

Sgt. Ken Castro of the Sarasota Police Department sent emails to members of the North Port Police Department in April 2009, informing them that the US Marshals Service requested that police not disclose StingRay in court depositions, but instead refer to it as "information from a confidential source regarding the location of the suspect".

The North Port Police Department implemented the new rules and continues to avoid mentioning StingRay, according to the emails.

When contacted by the Wall Street Journal, the US Marshals Service refused to comment.

Illegal telecoms equipment

There is an underground market for illegal telecoms equipment, much of which comes from China. The equipment can be used to pretend to be a mobile base station, tricking all users' phones to connect to it as they pass close to areas where the fake base station is located.

In addition to transmitting a mobile phone user's location and identifying information, data from text messages can easily be intercepted using an SMS server, without the user's mobile operator or the user themselves being any the wiser.

In March, Chinese authorities arrested a total of 1,530 suspects in a crackdown on spam text messages, and found that there were 3,540 cases of suspected crimes, with some of the scams run by gangs in various cities.

Around the same time, local TV station Sacramento News 10 submitted public information requests to all the major law enforcement agencies in Northern California.

StingRay in use across California

By piecing together information from a 2012 federal grant application from the San Jose police department, the TV station discovered that StingRay has been used by the San Jose Police, US Marshals Service, Oakland Police Department, Sacramento Sheriff's Department, San Diego Sheriff's Department, Los Angeles Police Department and the Los Angeles Sheriff's Department.

"We will work with the Fusion Center to partner with San Francisco and Oakland to ensure we have the ability to cover all of the Bay Area in deploying cell phone tracking technology in any region of the Bay Area at a moment's notice," the application said.

The San Jose Police Department said that it had consulted with the Sacramento Sheriff's Department about how it was using StingRay.

News 10 spoke to the Sacramento District Attorney's Office and was told that no judge had approved warrants using the device. Warrant applications filed by the Sacramento Police Department also showed that the police department had never divulged that it was using the device to track suspects.

None of the police departments mentioned have admitted to using the devices, and the worry is that a wide amount of data is being collected about citizens who are not under suspicion of committing a crime.