Colorado’s first litter of gray wolf pups since the 1940s has grown to include six pups.

Colorado Parks and Wildlife said Thursday that staff spotted the pups living in a den with two collared wolves known as John and Jane in northern Colorado, KCNC-TV reports.

The agency first announced June 9 that staff had spotted three pups in the pack.

The discovery comes after Colorado voters narrowly approved a ballot measure last year that requires the state to reintroduce the animal on public lands in the western part of the state by the end of 2023.

Gray wolves were hunted, trapped and poisoned into extermination in Colorado in the 1940s.

Officials last year confirmed the presence of the small pack of wolves in northwestern Colorado after a number of sightings since 2019. The animals were believed to have come down from Wyoming’s Yellowstone National Park.

Wildlife advocates see reintroduction in Colorado as a vital step in restoring the wolf more quickly to habitat stretching from the Canada to the Mexico border. Wolves were reintroduced in the Northern Rockies in the 1990s, and some 3,000 of the animals now roam portions of Montana, Wyoming, Idaho, Oregon, Washington and Northern California.