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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fast-flowing Mill Creek in San Bernardino Mountains

The children were playing with their mother near Mill Creek at the Thurman Flats Picnic Area on Highway 38 in the San Bernardino Mountains, authorities said. (FOX11 Los Angeles KTTV)

"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.

fast-flowing Mill Creek in San Bernardino Mountains

Search and rescue teams found the children more than an hour later near the river's edge. The siblings were pronounced dead at a hospital. (FOX11 Los Angeles KTTV)

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.

fast-flowing Mill Creek in San Bernardino Mountains

The mother searched for the missing children and enlisted the help of another family in the area before authorities arrived. (FOX11 Los Angeles KTTV)

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

fast-flowing Mill Creek in San Bernardino Mountains

A 2-year-old boy and his 4-year-old sister died after falling into a fast-flowing river on Tuesday afternoon in California's San Bernardino Mountains. (FOX11 Los Angeles KTTV)

No further details were immediately provided.

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