A rare white bison calf, celebrated by Native American communities since its birth in Yellowstone National Park, has not been sighted since June, according to the National Park Service.

The white bison calf was born on June 4 in Lamar Valley and was photographed by visitors and photographers. The pictures were shared online, winning the hearts of social media users and Native American tribes who hold the animal sacred.

“To date, park staff have been unable to locate the calf,” the National Park Service announced on its website Friday. “To our knowledge, there have been no confirmed sightings by park visitors since June 4.”

According to the park service, the calf is leucistic, not albino. Leucistic animals, like this calf, have black eyes and hooves with some pigmentation, the park service explained.

Describing the calf’s birth as a “rare natural phenomenon,” the park service noted that a similar event occurred in the late 19th century, before bison were nearly extinct. The birth is believed to be a 1-in-a-million occurrence, and perhaps even less frequent.

The birth of the calf in June could be due to a “natural genetic legacy” present in Yellowstone’s bison. The wild bison population in Yellowstone has slowly been rebuilt, the park service stated.

The bison population typically ranges from 3,000 to 6,000 animals divided into two subpopulations. The northern herd breeds in Lamar Valley and on the surrounding high plateaus, while the central herd breeds in Hayden Valley, according to the park service.

Each spring, about 1 in 5 bison calves die after birth due to “natural hazards,” the park service added.

Native American people have passed down a tale about the white buffalo calf for generations, the National Park Service shared.

During a famine, a Lakota chief sent scouts to hunt for food. The scouts spotted a figure and approached, discovering it was a woman. One scout, driven by desire, approached her despite the second scout’s warning that she was sacred.

A cloud surrounded the man and the woman, and he turned into a pile of bones, the park service recounted. When the second man approached, the woman told him she was wakan, or holy, instructing him to inform his people of her arrival.

When she arrived, she brought the white buffalo calf chanupa, or pipe, considered “the most sacred object a person can possess,” the park service detailed. She taught the tribe the seven sacred ways to pray and promised to return, bringing harmony and spirituality to a world in need.

Rolling around the earth four times, she transformed into a white buffalo calf and disappeared. After her departure, herds of buffalo surrounded the people.

Native American people now believe that the birth of a white buffalo calf signifies their prayers being heard and the fulfillment of prophecy.

“To American Indians, a white buffalo calf is the most sacred living thing on earth,” the park service wrote. “Some American Indians say the birth of a white calf is an omen, often occurring in unexpected places among the poorest people.”

Soon after the white bison calf was born in June, Native American community members shared their excitement and welcomed the animal in a ceremony held on Wednesday.

Chief Arvol Looking Horse, speaking at the ceremony, referred to the birth as “the second coming of the white buffalo calf.”

“We need to protect the white animals,” he emphasized at the event.

The calf was named “Wakan Gli,” meaning “Return Sacred” in Lakota, as reported by the Associated Press.