AMNA NAWAZ: The mystery of two missing monkeys and the death and disappearance of other animals from the Dallas Zoo has captured the country's attention over the last couple of weeks.

And, today, at the Houston Zoo, zookeepers are on high alert after a cut was discovered in the mesh fence around the pelican habitat.

As Stephanie Sy reports, authorities are still trying to pin down exactly what happened.

STEPHANIE SY: The series of unfortunate events at the Dallas Zoo began on January 13, when a clouded leopard named Nova escaped.

Police were called after zoo staff notice a suspicious opening in the big cat's enclosure.

SGT.

WARREN MITCHELL, Dallas Police Department: It was their belief and it is our belief that this was an intentional act.

And so we have started a criminal investigation.

STEPHANIE SY: The 25 pound leopard was found hours later on zoo grounds, but, that same day, a second breach was discovered in a fence used to enclose the zoo's langur monkeys.

None of them got out.

Then, on January 21, a lappet-faced vulture named Pin was found dead and wounded.

In a statement, the Dallas Zoo said: "The circumstances of the death are unusual, and the death does not appear to be from natural causes."

Concerns about criminal mischief have zoos on high alert.

ED HANSEN, CEO, American Association of Zoo Keepers: The zoos across the country have really taken a long, hard look at this.

STEPHANIE SY: Ed Hansen is the CEO of the American Association of Zookeepers, which is dedicated to advancing animal care and promoting zookeeper education.

ED HANSEN: Zoos all over the country are retraining their employees, taking extra security precautions, installing more cameras, basically whatever it takes.

STEPHANIE SY: The Dallas Zoo installed additional cameras and increased on-site security patrols in mid-January, but it wasn't enough.

Late last month, more than a week after the vulture died, two emperor tamarin monkeys, Bella and Finn, went missing, their enclosure also apparently sabotaged.

A search for the monkeys ensued, with police circulating an image of a suspect throughout North Texas.

The man was later identified as 24-year-old Davion Irvin.

KRISTIN LOWMAN, Dallas Police Department: Preliminary investigation and help from the public identified him as the man that we were looking to speak to regarding this case.

On Thursday, we received a tip that Irvin was seen at the Dallas World Aquarium near some animal exhibits.

STEPHANIE SY: Irvin has been charged with six counts of animal cruelty, in connection to the monkey heist at the Dallas Zoo and two counts of burglary.

According to court documents, Irvin told authorities he loves animals, and that, if released, he would steal more.

Bella and Finn were found in a closet in an abandoned home in Lancaster, a suburb of Dallas roughly 15 miles from the zoo.

Beyond losing weight, they showed no sign of injury.

Meanwhile, in Louisiana the search for 12 squirrel monkeys continues after they were stolen from Zoosiana on January 28.

The private zoo is located just outside Lafayette.

Zookeeper Ed Hansen says accredited institutions like the Dallas Zoo and Zoosiana are required to meet safety standards, but that, broadly speaking, security measures at zoos are primarily to protect people, rather than animals.

ED HANSEN: They're not meant to combat a break-in and someone who wants to do malicious harm to either the animal itself or the enclosure itself.

You know, in the future now, with this happening, maybe there will be some revisions to protocols.

STEPHANIE SY: No motive has been released in the Dallas Zoo case.

And the mysterious death of the 35-year-old vulture, an endangered species, remains under investigation.

For the "PBS NewsHour," I'm Stephanie Sy.