A 2-year-old California boy and his 4-year-old sister are dead after they were swept away in the swift waters of a river while playing with their mother on Tuesday afternoon, authorities said.

The mother and her children were playing near Mill Creek while spending the afternoon at the Thurman Flats Picnic Area on Highway 38 in the San Bernardino Mountains, the San Bernardino County Sheriff’s Department said. 

The mother was attending to her son at around 3 p.m. when her daughter was swept downstream, according to a statement from the sheriff’s department.

After the mother searched for the girl, the department said she returned and found her son also missing. The boy was also presumed to have fallen into the rapid water and taken downstream.

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"After frantically searching, she hiked up to the picnic area and contacted another family to help search for the children but were unsuccessful in locating them," the sheriff’s department said.

Fire and law enforcement officials were then called to help search for the two missing children.

Authorities found the 4-year-old girl around 4:40 p.m. near the river’s edge. The boy was also found near the river’s edge just after 5 p.m., according to the sheriff’s office.

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The siblings were rushed to local hospitals. Despite extensive life-saving measures, both children were pronounced dead.

No further details were immediately provided.

Authorities asked anyone with information about the incident to contact the San Bernardino County Sheriff’s Department Yucaipa Station at (909) 918-2305.