The 18-year-old was taken to hospital in a serious condition
The body was discovered five days after the 33-year-old Rivera disappeared on Lake Piru, where her son was found July 8 alone a few hours later on a boat the two had rented, the Ventura County Sheriff's Office said.
Authorities would not immediately say if the person found was Rivera, but said the day after she disappeared that they believed she drowned in the lake northwest of Los Angeles. A 2pm press conference was scheduled.
The lake an hour's drive from Los Angeles was searched by dozens of divers working in waters with little visibility, with help above from helicopters, drones and all-terrain vehicles.
Rivera played singing cheerleader Santana Lopez for six seasons on the Fox musical-comedy "Glee."
If she is declared dead, she will become the third major cast member from the show to die in their 30s.
Cory Monteith, one of the show's leads, died at 31 in 2013 from a toxic mix of alcohol and heroin.
And co-star Mark Salling, who Rivera dated at one point, killed himself in 2018 at age 35 after pleading guilty to child pornography charges.
Rivera had experience boating on the lake in Los Padres National Forest, authorities said.
The boy was wearing a life vest, and another life jacket was found in the boat along with Rivera's purse and identification.
Rivera is believed to have drowned "in what appears to be a tragic accident," the statement said.
The boy, Rivera's son from her marriage to actor Ryan Dorsey, was safe and healthy and with family members, authorities said. The couple finalized their divorce in June 2018 after nearly four years of marriage.
The most recent tweet on Rivera's account, from July 7, read "just the two of us" along with a photo of her and her son.
The 18-year-old was taken to hospital in a serious condition
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