A second monkey has been shot and killed while a search for a third missing monkey continues a week after the animals’ escape from a truck that overturned on a Mississippi highway.

Someone shot the monkey after seeing it cross the highway on Monday evening, about a mile from the scene of the 28 October crash, Randy Johnson, the Jasper county sheriff, said. Johnson said he was contacted by a person with the transport company who recovered the monkey after a civilian shot it.

The Mississippi department of wildlife, fisheries and parks confirmed Tuesday in a news release that one monkey was still unaccounted for after two of the escaped monkeys were “recovered deceased”. Officials have warned that people should not approach the Rhesus monkeys, saying they are known to be aggressive.

Over the weekend, a woman who said she feared for the safety of her children shot and killed another escaped monkey after her 16-year-old son saw a monkey outside their home near Heidelberg. Jessica Bond Ferguson said she and other residents had been warned that the escaped monkeys carried diseases – including herpes – so she shot it.

A truck carrying 21 monkeys overturned on Interstate 59 north of Heidelberg, and several monkeys escaped. Video from the scene showed monkeys and wooden crates in tall grass. Searchers in protective equipment were seen scouring nearby fields and woods for missing primates. Five monkeys were killed during the search, and three were missing initially, officials said.

The monkeys had been housed at the Tulane University National Biomedical Research Center in Louisiana, which routinely provides primates to scientific research organizations, according to the school. Tulane has said it wasn’t transporting the monkeys, and they do not belong to the university. The remaining 13 monkeys arrived at their original destination, according to Tulane.

PreLabs, which describes itself on its website as a biomedical research support organization, said in a statement Monday that a vehicle transporting the primates was involved in the crash – and the animals were being lawfully transported to a licensed research facility. It stressed that the monkeys weren’t carrying any known diseases but asked the public not to approach them as they were likely frightened and disoriented.

“We are cooperating with authorities and reviewing all safety procedures to ensure the continued well-being of both the animals and the community,” PreLabs said.

The escape is the latest glimpse into the secretive industry of animal research and the processes that allow key details of what happened to be kept from the public.