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The mystery behind why five monkeys were killed by police after a truck carrying the primates overturned in Mississippi has been solved -- and TMZ has the full scoop!

Jasper County Sheriff Randy Johnson tells us ... he contacted one of his deputies who responded to Tuesday's truck accident along a highway in rural Heidelberg, and he asked the deputy what kind of threat the monkeys pose to the public. Jasper County Sheriff's Department

Johnson said the deputy had the 2 truck occupants contact their supervisor -- who warned the cargo contained 21 test monkeys infected with COVID, herpes, and hepatitis C -- and, as a result, they needed to be handled with protective gear.

The supervisor also recommended the monkeys be put down ... and that's why police "destroyed" the

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