BOISE, Idaho (AP) — A break-in at Zoo Boise early Saturday left a Patas monkey dead from blunt force trauma to the head and neck and police were analyzing blood found at the scene to determine if it came from the monkey or one of two human intruders.

Two males wearing dark clothing were spotted by a security guard at 4:30 a.m. outside the fence near the primate exhibit, police said. Both fled, one of them heading into the interior of the zoo. Boise police used a thermal imager in searching the 11-acre zoo grounds but didn't find the person.

Police said late Saturday that a grey baseball cap with a distinctive skull design found near the site was probably left behind by one of the intruders and it might help in tracking them down.

"I've been here for 15 years and we haven't had anything like this happen," Zoo Boise Director Steve Burns said. "It's unfortunate that we have to let kids know that something like this happens. Monkeys are always among the most favorite animals here."

Patas monkeys, often called the military monkey, have reddish-brown fur with grey chin whiskers and distinctive white moustaches. They are widely distributed across central Africa south of the Sahara Desert and can live more than 20 years in captivity.

During a search of the zoo before dawn, Burns heard a groan that at first he thought sounded human. It turned out to be an injured Patas monkey barely moving near the perimeter fence.

The zoo's veterinarian was called, but the monkey died just before 6 a.m. as it was being examined. A necropsy later determined that blunt force trauma was the cause of death, police said.

An inventory done by zoo staff found no other animals missing or injured. The zoo has one remaining Patas monkey — another male — but it's unclear if it will remain at the zoo or will be sent to another zoo where it can socialize with other Patas monkeys, Burns said.

"They're not endangered in the wild, but there are not many in zoos in the United States," he said. "Monkeys are social animals. We only have one."

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The two Patas monkeys came to Zoo Boise about three years ago from Tampa's Lowry Park Zoo in Florida. They had an outdoor enclosure during the summer in Boise but were moved indoors to the primate building when colder weather arrived.

Burns said the monkeys hadn't been given names, and he didn't know their ages. The monkey that was killed was about 2 ½ feet tall and weighed about 30 pounds, Burns said.

Burns declined to discuss details of the police investigation, including how the intruder entered the primate building, if the monkeys might have been specifically targeted, or how the monkey ended up near the perimeter fence. The zoo doesn't have surveillance cameras, he said.

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

Amy Stahl of Boise Parks & Recreation said the death shocked zoo workers.

"They're hit hard," Stahl said. "They care for the animals on a daily basis and they care about them deeply."

The zoo was supposed to open at 10 a.m. but remained closed while police gathered evidence, opening about 2:30 p.m.