In a dramatic turn of events, one of the monkeys that fled following a truck accident on a Mississippi highway was fatally shot on Sunday morning. A local woman, concerned for her children’s safety, took matters into her own hands.

Jessica Bond Ferguson recounted how her 16-year-old son alerted her early Sunday, claiming to have spotted a monkey in their yard near Heidelberg, Mississippi. Responding to the potential threat, she armed herself with a firearm and her cellphone before stepping outside, where she noticed the monkey approximately 60 feet (18 meters) away.

Ferguson mentioned that local authorities had cautioned residents about the health hazards posed by the escaped monkeys. Acting on this information, she decided to use her gun.

The Jasper County Sheriff’s Office later confirmed in a social media update that a resident had encountered one of the escaped monkeys on their property that morning. The office noted that the Mississippi Department of Wildlife, Fisheries, and Parks had taken custody of the animal, although further details were not disclosed.

The Rhesus monkeys had been housed at Tulane University’s National Biomedical Research Center in New Orleans, Louisiana. The university, which routinely supplies primates to research institutions, clarified that the monkeys did not belong to them and were not being transported under their auspices at the time of the escape.

A truck carrying the monkeys overturned Tuesday on Interstate 59 north of Heidelberg. Authorities have said most of the 21 monkeys were killed. The sheriff’s department has said animal experts from Tulane examined the trailer and had determined three monkeys had escaped.

The Mississippi Highway Patrol has said it was investigating the cause of the crash, which occurred about 100 miles (160 kilometers) from the state capital, Jackson.

Rhesus monkeys typically weigh about 16 pounds (7.2 kilograms) and are among the most medically studied animals on the planet. Video recorded after the crash showed monkeys crawling through tall grass beside the interstate, where wooden crates labeled “live animals” were crumpled and strewn about.

Jasper County Sheriff Randy Johnson had said Tulane officials reported the monkeys were not infectious, despite initial reports by the truck’s occupants warning that the monkeys were dangerous and harboring various diseases. Nonetheless, Johnson said the monkeys still needed to be “neutralized” because of their aggressive nature.

The monkeys had recently received checkups confirming they were pathogen-free, Tulane said in a statement Wednesday.

About 10 years ago, three Rhesus macaques in the breeding colony of what was then known as the Tulane National Primate Research Center were euthanized after a “biosecurity breach,” federal inspectors wrote in a 2015 report. The breach involved at least one staff member failing to adhere to biosafety and infection control procedures, it said.

The facility made changes in its procedures and retrained staff after that happened, according to the report from the U.S. Animal and Plant Health Inspection Service.

Rhesus macaques “are known to be aggressive,” according to the Mississippi Department of Wildlife, Fisheries and Parks. It said the agency’s conservation workers were working with sheriff’s officials in the search for the animals.

The search comes about one year after 43 Rhesus macaques escaped from a South Carolina compound that breeds them for medical research because an employee didn’t fully lock an enclosure. Employees from the Alpha Genesis facility in Yemassee, South Carolina, had set up traps to capture them.