A 4-year-old girl and a 2-year-old boy died after being swept away by a river Tuesday in San Bernardino County, authorities said.

MENTONE, Calif. (KABC) — A 4-year-old girl and her 2-year-old brother died after being swept into a river Tuesday in San Bernardino County while out with their mother, authorities said.

The incident happened around 3 p.m. in Mill Creek near the Thurman Flats picnic area on Highway 38 outside of Mentone.

The San Bernardino County Sheriff’s Department said the mother was caring for her son when her daughter was taken downstream.

“The mother searched for her but was unable to locate her,” a statement from the sheriff’s department said. “When she returned, her son was missing and it was assumed he was also downriver.”

The mother then walked to the picnic area and contacted another family member for help, but the family member was unable to find the children.

Authorities said it took the mother about an hour to call 911 due to limited cell service in the area.

Deputies from the Yucaipa Station, Sheriff’s Air Rescue, Mentone County Fire Department, a U.S. Forest Service rescue team and SBCSD search and rescue members worked together in the search.

Crews initially found the girl and rushed her to a local hospital. Soon after, they found the boy. They were both found near the river’s edge, authorities said.

The girl was sent by ambulance to Redlands Community Hospital while the boy was airlifted to Loma Linda University Medical Center.

Their deaths were pronounced later that evening.

The incident remains under investigation.