Prosecutors in St Louis, Missouri, have dropped a criminal robbery case to avoid revealing details of a controversial mobile phone surveillance program, a defense attorney has claimed.

The St Louis Post-Dispatch reported that the state dropped more than a dozen charges against three defendants just one day before police were scheduled to testify as to how they were able to pull data from the accused's mobile phones.

Public defender Megan Beesley, who represented the accused, told the paper she was convinced the charges were dropped so that the officers would not be forced to disclose information about the secretive phone tapping devices used in the investigation, which are often known by the brand name StingRay.

Prosecutors have disputed that claim. But if true, it would add credence to previous accusations that law enforcement agencies hold the secrecy of the StingRay surveillance program above even putting offenders behind bars.

StingRay devices have been a poorly kept law enforcement secret for years. The suitcase-sized gadgets allow authorities to spoof a mobile base station in order to collect locational information on nearby phones. But police have reportedly been instructed by the FBI to keep details on the devices secret, perhaps even at the expense of criminal cases.

The St Louis case would be one of the few instances, however, in which such a policy was applied so publicly.

The case in question centered around accusations that the three suspects (two men and a woman) had engaged in a two-hour-long armed robbery spree in 2013, during which they had taken money and phones from seven victims, one of whom was pistol-whipped and required 18 stitches.

Prior to dropping the charges, police were scheduled to testify as to how they they were able to track the location of one of the stolen phones to a hotel room occupied by the suspects. Two of the suspects' charges were subsequently dropped and a third suspect who took a plea bargain is now seeking to change her guilty plea. ®