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Luna, Cayenne, Spice Girl, Sweet William and Maybelline — four miniature horses and a Sicilian donkey that had delighted countless children at San Jose’s Happy Hollow Park and Zoo — were mauled to death by dogs that got into their pen overnight twice last week, city officials said Tuesday.

“We’re just heartbroken,” zoo Director Valerie Riegel said outside Sweet William’s empty pen. “Everyone’s feeling the loss of these animals.”

The first attack came after the zoo closed Jan. 14, said Shannon Heimer, the zoo’s acting general manager. Zoo maintenance crews arrived the next morning to find Sweet William dead in his pen. A necropsy revealed later that day that the miniature horse had been mauled to death.

Security cameras captured grainy images of a pack of three dogs — two large and one small — running around the park grounds after hours, Heimer said. Zoo officials scoured the perimeter fencing that day but saw nothing amiss, Heimer said. However, a vehicle access gate behind the park’s roller coaster had a space that looked big enough for a dog, so zoo staff closed the gap.

The security cameras did not capture the attack itself because they aren’t focused on the pens, said zoo officials, who concluded that the dogs must have leaped over the four-foot tall fence around Sweet William’s enclosure.

Zoo staff set animal traps around the park and caught raccoons, a skunk and feral cat, Heimer said. They moved the sheep and goats into a barn and the rest of the horses and the donkey into a grazing pen surrounded by a six-foot chain-link fence.

There was no sign of the dogs again until the morning of Jan. 20, when maintenance workers arrived to find Maybelline the donkey and remaining horses dead inside the grazing pen, along with one large dog — a Belgian Malinois — that was running about inside.

“It was active, but not aggressive,” Heimer said, adding that the staff were able to coax it out with a treat and put a leash on it.

The zoo staff members surveyed the perimeter again and this time found signs that the dogs had dug under the fencing around the park and the grazing enclosure, Riegel said. Park staff believe they found the second dog seen in the surveillance video roaming nearby Kelly Park with four feet of loose, tattered rope around its neck that suggested it had been tied to a tree.

“We feel an immense amount of grief and sorrow,” Angel Rios Jr., director of Parks, Recreation and Neighborhood Services, which runs the park, said in a Jan. 20 statement posted on the zoo’s website.

He said the park and zoo remain open to the public while the city’s Animal Care and Services department investigates the attack, though they were closed as usual Monday and Tuesday during the off-season.

Julie St. Gregory, a spokeswoman for Animal Care and Services, identified the dog found in the pen as a Belgian Malinois and said the other dog looked similar. A third dog, which she believes to be a Chihuahua, has not been found, but “is not considered a threat.”

Rios said that two of the miniature horses and the donkey were born at the zoo.

Parents whose children came to adore the donkey and horses took to Facebook to mourn their loss, assail the dogs’ unknown owner and question how this could happen.

“This is incredibly sad,” posted Lindsay Marshall on the San Jose Parks Advocates’ Facebook page. “These animals were such a joy to my young son when we lived in San Jose. There sure as hell better be an investigation as to how these animals weren’t protected.”

Happy Hollow Park and Zoo opened in 1961 and underwent a $72 million renovation in 2008. The park features children’s rides, amusements, a puppet theater, play areas and an accredited zoo with more than 150 animals, including endangered lemurs, meerkats, a jaguar and a giant anteater.

The Malinois found in the pen, and likely the other dog, will probably have to be euthanized, St. Gregory said.

“We know the dog did this, so we can’t in good faith put this dog out to the public,” St. Gregory said.

If an owner steps forward, the owner could appeal to a hearing officer in a court proceeding for the animal to be spared, she said, but that seldom happens because the costs of securing a dog deemed dangerous are prohibitive. The owner also could face civil liability for the deaths of the zoo animals, she added.

Happy Hollow is accredited with the Association of Zoos and Aquariums, a nonprofit that promotes conservation, education, science and recreation at more than 230 U.S. and overseas institutions. Accreditation aims to ensure that the facility meets the AZA’s standards for animal management and care, living environments, social groupings, health and nutrition.

AZA spokesman Rob Vernon said Happy Hollow notified the organization last week after the attacks. The association is awaiting a report from the city about what happened before it decides whether any recommendations are warranted, he said.

Riegel said the zoo is planning to beef up security, but it was unclear if and when the beloved horses and donkey might be replaced.

“We’re giving the staff some time to process the whole thing before we go down that road,” Riegel said. “The community support has been tremendous. It means a lot to the staff.”