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

MENTONE, Calif. (KABC) — Authorities say a 4-year-old girl and her 2-year-old brother died after they were swept away in a river in San Bernardino County on Tuesday during an outing with their mother.

The incident occurred around 3 p.m. in Mill Creek near the Thurman Flats picnic area on Highway 38 outside 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 find her,” the sheriff’s department said in a statement. “When she returned, her son was missing and was suspected to be downstream as well.”

The mother then wandered to the picnic area and asked another family member for help, but she could not find the children.

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

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

The emergency services initially found the girl and took her to a local hospital. Shortly afterwards they found the boy. According to authorities, both were found near the riverbank.

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

They were pronounced dead later that evening.

The incident remains under investigation.