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.

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.

Turns out, though, the monkeys did not have any diseases, and the sheriff corrected the record later in the day after Tulane University — where the primates were transported from — issued a statement saying the animals were not infectious.

In addition to the 5 dead monkeys, Johnson said 13 others survived the crash and were corralled by the authorities … while 3 others escaped into the wild.

Johnson said the authorities are now looking for the 3 monkeys in dense woods, laying down traps with food in them. Johnson also said he’s concerned the monkeys might end up in someone’s backyard and bite a person trying to capture them.

Meanwhile, Tulane University sent us an updated statement, saying 21 rhesus macaques were being transported from the Tulane National Biomedical Research Center when the truck crashed.