A short-lived hoax on Twitter briefly erased $200 billion of value from U.S. stock markets on Tuesday, underscoring the vulnerability of financial markets to computerized trading programs that buy and sell shares without human intervention.

A tweet purportedly from the Associated Press just before 1:08 p.m. reported two explosions in the White House and that President Barack Obama had been injured. The posting sent the Dow Jones Industrial Average tumbling roughly 145 points in an instant.

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