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not to be confused with the Alias television series episode Dead Drop

A dead drop or dead letter box, is a location used to secretly pass items between two people, without requiring them to meet.

Spies have been known to use dead drops, using various techniques to hide items (such as money, secrets or instructions) and to signal that the drop has been made.

The system involves using signals and locations which have been agreed in advance. These signals and locations must be common everyday things to which most people would not give a second glance.

The location (the Dead Letter Box) could be a loose brick in a wall, a library book, a hole in a tree or a shrub in a park etc. It should be something common and from which the items can be picked up without the operatives being seen by a member of the public or the security forces who may be watching them.

The signaling devices can include a chalk mark on a wall, a piece of chewing-gum on a lamppost, a newspaper left on a park bench etc. For example, Aldrich Ames left chalk marks on a mail box located at 37th and R Streets NW in Washington, DC to signal his Russian handlers that he had made a dead drop. The number of marks on the box prompted some local residents to speculate, somewhat jokingly, that it was used by spies.

The dead drop is often used as a cut-out device. In this use the operatives who use the device to communicate or exchange materials or information do not know one another and should never see one another. While this type of device is useful in preventing the roll up of an entire espionage network it is not foolproof. If the lower level operative is compromised he or she may reveal the location of and signal for the use of the dead drop. Then the counter espionage agents simply use the signal to indicate that the dead drop is ready for pickup. They then keep the spot under continuous surveillance until it is picked up. They can then capture the operative who picked up the material from the dead drop.

Dead drop spike

The dead drop spike is a concealment device similar to a microcache which has been used since the late 1960s to hide money, maps, documents, microfilm, and other items. The spike is waterproof and mildew-proof and can be shoved into the ground or placed in a shallow stream to be retrieved at a later time.

On January 23, 2006, the Russian FSB accused Britain of using wireless dead drops concealed inside hollowed-out rocks to collect espionage information from agents in Russia. According to the Russian authorities, the agent delivering information would approach the rock and transmit data wirelessly into it from a hand-held device, and later his British handlers would pick up the stored data by similar means.^{[citation needed]}

A British al-Qaeda related terrorist cell, convicted of terrorist offences on April 30, 2007, used an email account as a dead drop technique. By sharing the account's password, members of the group could write messages for each other and save them as drafts, without leaving a permanent recording by actually transmitting them.^{[citation needed]}

• "Russians accuse 4 Britons of spying" News report on Russian discovery of British "wireless dead drop"
• "Old spying lives on in new ways", BBC, 2006-01-23