One of the oddest episodes in broadcast history happened on this day in 1971. An early warning system used by the US government was wrongly activated, causing TV and radio stations across the country to alert the public to an emergency that did not actually exist.

The Emergency Broadcast System (EBS), in use since 1963, was designed to allow the sitting president to communicate directly with Americans in the event of a major threat to the country or a national disaster. Or, as the United States Defense Civil Preparedness Agency put it, it was "an expeditious method of communicating with the American public in the event of war, threat of war, or grave national crisis".

When activated, EBS operators would send a teletype message to certified broadcasters and news agencies to transmit nothing but information related to the emergency. The command was also a signal for non-certified stations to cease broadcasting immediately.

Authentication of this feared message was provided by a codeword which changed daily. If the two codes matched, it was not a test.

At 9.33am Eastern Standard Time (EST), an emergency message with the relevant authentication codeword - ‘hatefulness’ - was sent to all the nation’s broadcasters and main news agencies, ordering them to cease normal programming and air a national emergency alert.

Serious confusion reigned. An estimated 20% of outlets followed the correct emergency procedures and cleared the airways in preparation for a statement from the office of President Richard Nixon.

Others started the procedures but decided to stop before receiving an all-clear, and some ignored the alert completely as it was received at exactly the same time as the regular, scheduled tests.

Operators at the National Emergency Warning Centre facility at Cheyenne Mountain in Colorado soon realised their mistake but sent several cancellation messages without the relevant authentication codeword - ‘impish’. Finally at 10.13 EST, a message with the all-important cancellation codeword was sent.

Given the nation’s ongoing involvement in the war in Vietnam, and the fact that South Vietnamese troops had invaded Laos just a week before with US military support, most of those who heard the message had spent 40 minutes convinced that America was under attack.

Click on the play button below to hear the broadcast as heard by listeners of Indiana's WOWO Radio station. The voice belongs to broadcaster Bob Sievers. He later described the incident as his "longest five minutes in radio". 

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