MOUNTAIN HOME VILLAGE, Calif. (AP) — Two young siblings died after being swept away by a rapidly flowing creek in Southern California’s San Bernardino Mountains, authorities said.

The tragedy occurred Tuesday when a mother took her 4-year-old daughter and 2-year-old son to spend the afternoon at Thurman Flat picnic area on Highway 38 near Mill Creek, the San Bernardino Count Sheriff’s Department said in a news release.

They were playing along the creek when at one point the mother was attending to her son and her daughter was taken downstream, the department said.

“The mother searched for her but could not locate her,” the department said. “When she returned, her son was missing and assumed to be down river as well.”

The mother frantically searched for both children and then hiked up to the picnic area and asked another family to help search, an effort that was unsuccessful.

Rescue crews from multiple agencies arrived at the scene about 70 miles (113 kilometers) east of Los Angeles around 4 p.m. and eventually found both children. The siblings were taken to local hospitals and pronounced dead after extensive efforts to save their lives, the department said.

The names of the children and their mother were not immediately released.

California authorities regularly warn that the state’s rivers and streams can become very dangerous when spring snowmelt fills them with fast-flowing, very cold water.