A Grevy's zebra walks in his enclosure at the National Zoo last month. (Jacquelyn Martin/AP)

A 900-pound male zebra got through an unlocked stall gate into a holding yard where it attacked a veteran zookeeper, kicking and biting him, the National Zoo said Friday, calling the incident the result of human error.

Wayne Millner, a veteran zookeeper who has handled tigers and cheetahs and was about to retire, was attacked at 8:30 a.m. Nov. 18 by a male Grévy’s zebra that got out of its stall and into holding yard K, where the keeper was working alone.

Millner was severely injured and hospitalized. The zoo said he has no recollection of the incident, and there were no witnesses. But it said the keeper did not follow proper procedure in apparently leaving the gates open.

The attack happened in the zoo’s troubled Cheetah Conservation Station, where a series of other serious mishaps have occurred in recent months, including the death of a young gazelle that was apparently spooked by the attack and ran into a barrier.

A summary of an investigation by safety inspector Mary Lariviere was released Friday. “The zebra should have been contained in his primary stall while the keeper was cleaning the adjoining stall,” the zoo said in a separate statement.

Zebras are dangerous, and males have canine teeth, incisors designed for fighting and jaws like a giant pair of pliers.

The summary said there was a wheelbarrow, rake and shovel in the yard with Millner, suggesting that he might have been cleaning the area.

It said that all the gates between the zebra’s barn and the holding area were open, as was the gate from the holding area to the keeper walkway. The zoo said the zebra was not in danger of escaping into the public area.

The zoo said staff members and a volunteer keeper aide managed to rescue Millner by distracting the zebra and shifting it to a different secure location. Gumu, the zebra, was known to be especially aggressive, the zoo said.

The summary said there are protocols for handling the zebras, but there is no documented evidence of staff being trained in those protocols.

It said the causes of the incident were the keeper not following protocol, and the gates being left open.

The zoo said a zebra got into a holding yard last December, also the result of human error.

Millner is out of the hospital but has not returned to work. A woman who answered the door at his house Monday said he could not be interviewed.

The report recommends that keepers get individual protocol and first aid training. It suggests making grief counseling available for those involved in the attack. And it urges a zoowide workload analysis.

The report comes amid an internal zoo report, released Wednesday, that details other lapses at the Cheetah Conservation Station, including the death Dec. 17 of an African red river hog that died of severe weight loss and chronic skin and mouth infections.

Also Wednesday, the zoo announced the death of another animal under its care. A Przewalski’s horse was found dead in a barn at the zoo’s Conservation Biology Institute, a research site in Front Royal, Va.

The horse appears to have charged into a fence inside the barn, where it lived with its mother. Przewalski’s horses are critically endangered in the wild.

The 4-month-old colt was apparently spooked by something. It was found Wednesday morning, lying beside the fence, which was bowed outward. Preliminary necropsy revealed a traumatic neck fracture.

The report released Wednesday found that “animal care and overall organization, accountability, follow-up, and communication are severely lacking” in the cheetah complex. It recounted a broken boiler, a dirty heater, inexperience and poor communication.

It reviewed the death of a third animal, a skittish antelope-like kudu, which apparently was startled and rammed itself into a paddock wall, breaking its neck. The animal arrived at the zoo May 10 and was found dead June 16.

Zoo Director Dennis Kelly has said the lapses are indications of the zoo’s strapped finances and stretched staff.

The report was sparked by complaints in June from a worried volunteer who brought a series of concerns to Kelly and the associate director of animal care, Don Moore. They then set up a committee to conduct the inquiry.

That report, which was completed Sept. 3, also details the spring’s escape of a female non-flying vulture named Natalie, who apparently was able to use a wind gust to soar out of her enclosure.

A “code green” alert for an escaped animal was issued, and the vulture was caught moments later in an adjacent parking lot. The zoo said it now keeps the bird’s flight feathers better clipped and describes the incident as a “learning experience.”

In a later incident, a red panda escaped from the zoo in June by using overhanging tree branches and got some distance away before it was captured.

Moore, responding to the first report, disagreed that the zoo was “severely lacking” in care, organization, accountability and follow-up. He agreed that there were lapses but said that they had been addressed.