Officials say monkeys escaped captivity when a truck carrying them overturned on a Mississippi highway. The Jasper County Sheriff's Department says all but one of the escaped monkeys were killed, warning they were “aggressive.” It's unclear how many monkeys were in the truck. Video shows monkeys crawling through tall grass on the side of Interstate 59 just north of Heidelberg, Mississippi. The truck was carrying Rhesus monkeys, which are commonly used in medical research. The sheriff’s department initially said the monkeys were carrying diseases. Tulane University in New Orleans, which had housed the monkeys, says they were not infectious.