Dye pack

A dye pack is a radio-controlled device used by banks to foil a bank robbery by causing stolen cash to be permanently marked with dye shortly after a robbery. They were invented in Georgia, United States, in 1965.[1] In most cases, a dye pack is placed in a hollowed-out space within a stack of banknotes, usually $10 or $20 bills. This stack of bills looks and feels identical to a real one, but usually subtly marked in a way that is only privy to selected bank employees. The development of flexible dye packs makes it virtually impossible to detect by persons handling the stack.[2][3]

Deployed dye pack

When the marked stack of bills is not used, it is stored next to a magnetic plate near a bank cashier, in standby or safe mode, ready to be handed over to a bank robber by a bank employee. When it is removed from the magnetic plate, the pack is armed. Once it leaves the building and passes through the door frame, a radio transmitter located at the door triggers a timer (typically at least 10 seconds), after which the dye pack explosively releases[4] an aerosol (usually of Disperse Red 9) and sometimes tear gas, intended to destroy the stolen money and mark the robber's body with a bright stain. The chemical reaction causing the explosion of the pack and the release of the dye creates high temperatures of about 200 °C (392 °F) which further discourages a criminal from touching the pack or removing it from the bag or getaway vehicle.[2] Dye packs are used to foil robberies in over 75% of banks in the U.S.[2]

In England, a robber who fled a bank in Regent Street laughing "Haha, I've got all the money", was enveloped in "a red cloud" of dye as he ran away down Shaftesbury Avenue.[5] He was later caught and jailed.[5]