Updated at 9:53 p.m. ET: Police looking for two suspects who broke into a Boise, Idaho zoo and fatally beat a monkey have a clue: a light gray cap near where the monkey was found.

The hat is emblazoned with a white skull surrounded by swirly drawing and the logo "M L" written on the skull's forehead.

A security guard told officers he spotted two males in dark clothing, one inside the zoo grounds and the other outside the fence near the primate exhibit. When the suspects saw the guard, they ran.

Police searched the 11-acre zoo, including one sweep with a thermal imager, but did not find the suspects.

During the search, officers and zoo employees found a Patas monkey lying next to the perimeter fence near the primate exhibit where the suspects were last seen. The monkey appeared to have a head injury and died a short time later.

When they were searching for the offender, Burns said they heard a groan, the Idaho Statesman reported. It was unclear whether the groan came from a human or a monkey, he said.

The monkey shared a cage with another male monkey that was not harmed, according to the Statesman.

Detectives collected blood evidence at the scene that's being tested to determine if it's blood from the monkey or a human.

"It's very disturbing that someone would intentionally break into the zoo and harm an animal. We're doing all we can to find who did this." said Sgt. Ted Snyder of the Boise Police Department.

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“Everybody here at the zoo is devastated,” zoo Director Steve Burns said in a statement.

“It’s sad to have to tell kids that one of their favorite animals is gone.”

A veterinarian is conducting a necropsy of the monkey to determine the cause of death. All the other zoo animals were accounted for and uninjured.

Patas monkeys are ground-dwelling animals from the plains of Africa. At 2 ½ feet, they typically weigh 35 pounds, according to a zoo statement.

Patas monkeys are rare in zoos but are not endangered in the wild, says Burns. There is one remaining male Patas monkey at the zoo. Both of the animals came to Zoo Boise three years ago from the Lowry Park Zoo in Tampa Bay, Fla.

“Because monkeys are social animals we are concerned about the welfare of the remaining animal,” said Burns. The zoo will explore opportunities to replace the monkey or move the remaining animal to another zoo.

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