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A zookeeper left a door open at the Dallas Zoo and Tufani was able to escape.
But zoo officials say the 19-year-old ape remained in the gorilla research complex and never got into the zoo's public areas.
The zoo was closed at the time.
Dallas Zoo officials say they've suspended the zookeeper who failed to follow proper procedures.
Tufani was tranquilized and returned to the living area in less than an hour.
