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I've heard of bank robbers being foiled by a "dye pack" put in their money stash. What is a "dye pack"?

In a typical bank robbery, the robber hands a note to the bank teller listing his demands, usually instructing the teller to put money in a bag or other object. The dye pack device was invented as a way to non-violently render a bank robbery pointless by permanently staining the stolen money a bright red color, alerting everyone to the fact that the money being passed to them is stolen.

The dye pack used in over 75 percent of banks in the United States is called the "SecurityPac," made by ICI Security Systems. A dye pack consists of a stack of real bills, usually of $10 or $20 denominations, with the dye device stuck in the middle of the stack. In the past, the device itself was made of a rigid plastic and was quite detectable to the skilled criminal. Today, however, new technology has allowed the dye to be housed in a thin, flexible package, making a dye pack virtually indistinguishable from a regular stack of money.

Bank tellers have several of these packs near their station at all times. A pack is put in "safe" mode by attaching it to a special magnetic plate. During a robbery, a teller tries to slip one of the dye packs into the money bag without the thief noticing. While the thief is still inside the bank, the dye pack remains dormant. Within the dye package is a small radio receiver that is activated when the pack is removed from the magnetic plate. A small radio transmitter is mounted inside or near the door frame of all entrances of the bank. Once the dye pack passes through the door and receives the specific radio frequency signal, it activates. The dye pack is usually set on a timer of 10 seconds or longer so that the criminal is either in his getaway car or running a good distance from the bank before the package explodes.

When the dye pack explodes, it releases an aerosol of red smoke, red dye (1-methylamino-anthraquinone) and, in some cases, tear gas. When these chemical reactions take place, the package burns at a temperature of about 400 degrees Fahrenheit (204 degrees Celsius), discouraging any attempts to remove the device from the bag. (Further details of the chemical activation are "classified.") Typically, the explosion of the dye pack compels the thief to throw the bag, so the bank gets its money back. In addition, the red dye frequently stains the thief's clothes and/or hands, making identification of the suspect quite easy.

To date, the "SecurityPac" has helped to recover nearly $20 million and to apprehend about 2,500 criminals.

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