It’s been along time since I first saw in open court the power of the surveillance state. A client of mine was accused of rape. When the victim turned up dead, the state accused my client of killing her. Prove it, we said. The state did, with the help of cell tower evidence.

It wasn’t really much of a rape case. The victim and the accused were long-time lovers. As passions ebbed, boundaries became blurred. What was once an occasion for a joyous union became contested terrain. Did she yes, or no? Would it have mattered to him?

The case was, as criminal defense lawyers say, "triable."

Then she ended up shot dead. A single shot to the head. Someone heard the shot, and found her lying in a public park. She died almost immediately thereafter. Someone caught a glimpse of a car fleeing the scene; not much more.

The park in which she was found was in the northern part of the state. My client and she both lived along the southern coastline. She was north that fatal night to attend a softball game in a city park.

In a "he said/she said" kind of case, a dead witness is usually a gift, unless, of course, the accused killed the witness. In that case, all sorts of dire things happen, such as a waiver of the right to cross-examine the witness due to misconduct.

The state put on evidence. A murky federal official traveled up from Washington, D.C., to help connect the dots. He showed that my client was a heavy cell phone user. During a 30-hour period, the client had made hundreds of calls from his cell phone.

The witness then plotted those calls based on signals captured at cell towers, showing how the calls all originated along the shore, until an hour or so before the time of the murder. You could see the signals tracing a route northward, placing him at the park at the very time the murder took place. Moments after the shooting, you could see the calls heading south again. The calls, when plotted on a map of the state, defined a spike leading from south to north and peaking at just the wrong time and at just the wrong place. It was powerful evidence my client, or someone using his cell phone, was present at the murder scene at the very moment fatal shots were fired.

Cell tower evidence can greatly assist the state in showing the location of an accused person at any given point in time. It is a devastating tool in the state’s arsenal.

So it only seems fair that when such evidence tends to show innocence, a defendant ought to have access to it. In fact, the law requires a state or the federal government to turn over exculpatory evidence when they have it.

A South Florida man is trying his luck with just this theory.

Terrance Brown is accused of a series of bank robberies in the Fort Lauderdale area. He has raised an alibi defense, asserting, in effect, that he could not have committed the crimes because he was not near the banks.

To help prove his case, he has asked the government to produce cell phone records. The government contends that the service provider has destroyed the records. So sorry, the prosecution says. A private entity destroyed the records. There is nothing we can do.

Really?, lawyers for Brown said. With all the new disclosures about the National Security Agencies collection of phone records, why not just turn over data gathered by the nation’s super spooks?

Oh, that might raise national security concerns, the prosecution responds.

So let me see if I get that straight. The government can hoard data about us, and use it, too, when they choose to prosecute us. When we seek access to the data to vindicate the presumption of innocence, we’re told that might compromise national security. We’re kept safe from terrorists at the expense of our liberty.

Is it any wonder that Edward Snowden sought refuge in another country? •