The National Park Service is investigating after two wolf pups were struck and killed by a vehicle inside Yellowstone National Park.

The incident, which occurred at roughly sunset on Nov. 19, took place between Tower Junction and the Northeast Entrance. The wolves, black female and male pups, were from the Junction Butte Pack, one of the most frequently-observed packs in the park.

“Having studied these pups since birth, I believe their exposure to, and fearlessness of people and roads could have been a factor in their death,” said Yellowstone’s senior wolf biologist Doug Smith.

In a statement Wednesday, park officials say the pack of 11 adult wolves attended a den of pups near a popular hiking trail in the northeast part of the park during this past summer. The den and surrounding area were closed to the public in order to keep visitors and the wolves from having contact with each other.

When the pups approached the trail and got close to hikers, most of the people moved away. But some violated the required 100-yard minimum distance from wolves and approached the pups when they were on or near the trail, so that they could take a photograph.

Other people illegally entered the closed area to get near the wolves, park officials said. Having gotten used to the hikers, the pups then approached visitors along a road.

Park staff hazed the pups several times over the past five months in an effort to discourage them from approaching people and roads. But officials say the effort was never fully successful, and the pups "continued to demonstrate habituated behavior due to continued close encounters with visitors."

“Visitors must protect wolves from becoming habituated to people and roads. Stay at least 100 yards from wolves, never enter a closed area, and notify a park ranger of others who are in violation of these rules," Smith added.

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