ST. LOUIS • Police here in January 2014 used borrowed U.S. Secret Service cellphone tracking technology so high-tech that an agent would later boast that authorities could pinpoint a cellphone to within a foot of its actual location.

It did track a phone belonging to a St. Louis man, Melvin Dante Hardy, who was wanted on an arrest warrant and in connection with another investigation. The device allowed police to find and arrest Hardy, who federal prosecutors say has an “extensive criminal history.” He was charged a month later with drug crimes and resisting arrest.

Late last year, he was indicted on federal drug and gun charges that include evidence obtained after that arrest.

But now Diane Dragan, an attorney for Hardy, 31, says that some of the charges, and all of the evidence stemming from that arrest, should be tossed out of court because the cellphone tracking was illegal.

Some say police misled the judge by failing to tell her that they were going to use a cell site simulator, commonly known as a StingRay for one of the brands in wide use. The legal mechanism police employed to use the StingRay was inadequate, lawyers say, because federal prosecutors have now conceded that the use of the cell site simulator was a formal “search” under the law, requiring a search warrant.