“We’re presuming that an accident happened and we’re presuming she drowned in the lake,” Deputy Chris Dyer said at a news conference Thursday.
Search and rescue efforts for the actress have now shifted to a recovery operation, Dyer said.
“The goal is still to bring Miss Rivera home to her family,” he said.
Rivera, 33, vanished from Lake Piru on Wednesday evening. She went to the lake that afternoon and rented a pontoon boat with her 4-year-old son, according to authorities.
Dyer said Thursday that the boat was overdue for return around 4 p.m. local time. A rental employee found the boat on the north end of Lake Piru.
Rivera’s son was on the boat, but she was nowhere to be found, Dyer said. One life jacket was found on the boat and the boy was wearing another. Investigators found Rivera’s purse and her identification, but no other clues on the boat.
Search efforts began immediately and continued until about 10 p.m. Wednesday. The search resumed Thursday morning and will continue until sunset and resume at daybreak if necessary, the sheriff’s office said.
Video from the dock shows Rivera and her son as the only two people getting on the boat, Ventura County Sheriff’s Sgt. Kevin Donoghue said at a separate news conference.
Detectives have spoken to Rivera’s son and as a result of that conversation do not believe Rivera made it to shore, he said.
The boat was not anchored, and was not near the shore when found, the sheriff’s office said. The area in which it was found is approximately 30 feet deep.
The lake has very low visibility with numerous plants and trees in the search area, making it unsafe for divers and a more complicated search, according to Donoghue.
“If the body is entangled in something beneath the water, it may never come back up,” Donoghue said.
Another spokesman for the Ventura County Sheriff’s Office, Capt. Eric Buschow, previously said that the child and his mother went swimming and that she did not get back on the boat.
The child is healthy and with family, Dyer said Thursday morning. CNN has reached out to Rivera’s representatives for more details.
Rivera’s son is “doing as well as can be expected,”, according to Donoghue. “Our hearts and our prayers and thoughts go out to the Rivera family. This is a terrible tragedy for all of them and we hope to bring closure for that family,” he said.
The lake has been closed during the operation, which involves more than 100 people, Donoghue said.
Specialized dive teams from the nearby counties of Tulare, San Luis Obispo, and Los Angeles are also searching the lake. Others are using helicopters, boats and personal watercraft to look for clues.
Searchers are using sonar and other technology to assist in the recovery. For now, the search is focused on the north end of the lake.
“We don’t think it would have traveled the whole distance of the lake,” Donoghue said, explaining that they are looking where the boat was initially found and working their way outward from there.
“I think it’s too early to say what happened here. We’re still investigating and just still trying to find out what happened,” he said, noting that Rivera had been to this lake before.
Rivera played Santana Lopez on the Fox show for six years from 2009, and appeared in nearly every episode of the musical-comedy-drama. She was also on the CBS sitcom “The Royal Family” and in the comedy film “The Master of Disguise.”
Fellow celebrities, including her “Glee” co-star Harry Shum Jr., shared their concern on social media.
Actress Jackée Harry tweeted a scene she shared with Rivera on “The Royal Family,” which was Rivera’s first acting job at the age of 4.
“Prayers up for the lovely Naya Rivera,” Harry tweeted. “We starred alongside one another in her first very television series, The Royal Family, and I’ve watched her career blossom ever since. Please God, don’t cut this life short.”
Zack Teperman, who described himself as Rivera’s former publicist, said he was shocked by the news of her disappearance.
“I still can’t believe what I’m hearing and hoping it isn’t true,” he posted on social media. “I’m just sad. Naya Rivera was one of the first people I had the honor of working with back in 2009 when I got into public relations and moved to Hollywood. One of the first people to allow me to give her advice and guide her through those crazy Glee days.”