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Ventura County Sheriff's office said late on Wednesday it had identified the missing person as Rivera and a search and rescue operation at Lake Piru would continue.
The 33-year-old had rented a boat in the early afternoon along with her four-year-old son, CBS Los Angeles reported, citing the sheriff's department.
Her child was found alone in the boat about three hours later by an another boater, but was said to be unharmed, the report said.
The boy told investigators that he and his mother were swimming, but she never got back into the boat, it added.