(Reuters) - A bear cub believed to be the offspring of Grizzly 399, one of the best-known bears in the United States, has been hit by a car and killed in Grand Teton National Park in Wyoming, a park spokesman said on Monday.

The blond-faced cub was killed at about 10 p.m. on Sunday night near Pilgrim Creek Road at the national park, said Andrew White.

The driver of the vehicle did not report the incident involving the 50-pound (22.5 kg) cub to the park's dispatch center, White said.

Witnesses said Grizzly 399 frantically tried to attend to her injured offspring, the National Geographic reported.

Park officials are fairly certain the cub belonged to Grizzly 399, White said, but were waiting for DNA results to confirm the relationship.

"The death of this cub is especially tragic since Grizzly 399 is nearing the end of her reproductive life, and sadly she has only replaced herself in the population with one adult female, Grizzly 610," the Wyoming Wildlife Advocates conservation animal group said on its Facebook page.

A representative from the group could not be immediately reached for comment.

Grizzly 399 is 20 years old and is known for being particularly fertile, often giving birth to triplets. She is responsible for 16 descendants, but more than half have died in negative encounters with humans, according to the National Geographic.

The bear's latest cub, known as Snowy by the bear watchers of Grand Teton, was adored for its antics and distinctive white face, the group said.

Grizzly 399 - named for the number given to her by researchers who track her by a radio collar - rose to fame in 2006, according to the National Geographic, when she was first spotted by the roadside.

The bear is featured in a new book "Grizzlies of Pilgrim Creek" by nature photographer Thomas D. Mangelsen who spent nearly two years capturing Grizzly 399 and her extended family, the National Geographic reported.

Grizzly bears are listed as a threatened species in the United States, but there are proposals to delist them in some areas of the country where their populations have rebounded.

(Reporting by Justin Madden in Chicago; Editing by Fiona Ortiz and Sandra Maler)