A four-year-old boy and his eight-year-old sister were swept down the Kings River near Pine Flat Dam on Sunday.
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Crews later recovered their bodies.
Deputies say a few groups of people were in the area where the children entered the water.
Investigators want to speak with anyone who was at the river between 1 pm and 2 pm that Sunday.
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If you witnessed anything, you're urged to contact the Fresno County Sheriff's Office.