The recent escape of several research monkeys after the truck carrying them overturned on a Mississippi interstate is the latest glimpse into the secretive world of animal research. The escape is one of three in the U.S. over the past four years. Three monkeys remain missing since the truck crashed on Tuesday along Interstate 59 in Mississippi, spilling crates labeled “live monkeys.” Five of the 21 Rhesus macaques were killed during the search. Authorities haven’t disclosed the driver or company involved, or the monkeys’ destination. Tulane University confirmed the monkeys had been housed at its research center, but said it did not own them.