This article from RT simply states "An officer with the St. Louis Police Department’s Intelligence Unit had been scheduled to deliver a deposition under oath on April 9 concerning tactics that authorities relied on to identify and ultimately charge four suspects with 14 counts of first-degree robbery and armed criminal action regarding a 2013 crime spree." That is until the sudden dismissal of charges. Three of four involved in this case were set to go to trial, while one defendant already .
Of course publicly the prosecution denies a link but fails to give a justification. The Stingray is a spying device used to simulate cell-towers to track individuals. The Baltimore Sun was able to discover that SLPD was told not to “distribute, disseminate or otherwise disclose any information” without expressed written approval of the FBI. Apparently letting armed robbers go free is worth more than revealing the nature of these Stingray devices to the public.