Forget exploding dye packs. Three thieves who made off with about $9,000 in cash from an Illinois bank were thwarted by a GPS device inserted in the cash that led authorities straight to their door, according to the Chicago Tribune.

Timothy Rucker, 33, Phillip Griffen, 31, and Brandon Barnes, 25, entered a branch of the TCF Bank on Dec. 30 with their faces concealed and pointed a gun at a teller, demanding cash.

The three made off with a nylon bag full of money. But unknown to them, the bag contained two GPS-tracking devices hidden among the bills.

Signals from the devices led police to the home of one of the suspect's parents, where the thieves were arrested about an hour after the robbery.

Threat Level was unable to reach the bank to determine the make of the device it used. But it could have been a system such as the one made by 3SI Security in Pennsylvania, a leader in currency protection systems.

The company wouldn't answer any questions about its security systems. But according to its website, the GPS currency tracker it sells, called Electronic Satellite Pursuit (ESP), has helped recover more than $3.1 million.

In 2008, 3SI acquired Geotrax Protection, which developed a GPS tracking system for currency in 2002. This appears to be the system 3SI is marketing as ESP.

According to a highly detailed paper (.pdf) written in 2006 by Geotrax's founder, Richard Fuller and Phillip Grimm, the GPS tracking device had been deployed in more than 30 robberies as of November that year. Geotrax claimed at the time that the system had a recovery rate of more than 73 percent.

The device uses GPS, cell-tower tracking and RF beacons. It's inactive while sitting in a bank teller's cash drawer, with the power on low, and activates only when it moves outside of the reading field of a magnetic plate. At that point, it triggers an alert to security personnel and police by e-mail, pager or SMS to notify them that a pack is on the move.

In developing the currency tracker, the authors wrote that they needed a device that wouldn't be so obvious that it would be detected by the robbers within the first five to 10 minutes after a robbery.

"This 5-10 minute period is the crucial response time for the police to isolate the location of the criminal," they wrote. "Therefore, precise location as soon as a minute or two after activation is necessary to support effective response by the law enforcement community."

It also couldn't add significant weight to a stack of currency and needed to be flexible so that it would bend with the cash. And it needed to contain a battery that would last at least 45 minutes after activation, even after sitting in a teller's drawer for 18 months.

There were other concerns as well. The device had to be precise enough to locate a pin in a haystack, so to speak. If a thief placed it in a car in a parking lot, police would need to know which car contained the cash to obtain a search warrant. So the device includes an RF direction-finding beacon to help isolate it.

In one case in which the device was used, the currency was tracked for approximately five minutes through GPS before it stopped moving. The device remained stationary while two officers, using separate beacon receivers, walked through the neighborhood to isolate the signals. They eventually narrowed the location to a single house and then a paneled wall inside the house. The cash was sealed in the wall and was finally recovered about three and a half hours after the robbery.

(Hat tip: William Knowles)

Photo of Geotrax currency tracker courtesy of Geotrax (.pdf).