On Wednesday, the Glee actress, 33, was reported missing after a trip to Lake Piru in Ventura County, California. Rivera and her 4-year-old son, Josey — whom she shares with ex-husband Ryan Dorsey — rented a pontoon boat, but when she did not return it after the three-hour rental period, the staff at the lake found it at the north side of the body of water, with Josey alone and sleeping aboard.
Following his discovery on the pontoon, the little boy told investigators that his mother did not get back in the boat after being in the lake, a press release from the Ventura County Sheriff's Office revealed.
"Rivera’s son told investigators that he and his mother had been swimming in the lake, and he got back in the boat, but Rivera did not," reads the press release, which was released Thursday.
naya rivera/instagram Naya Rivera and son Josey
While a search-and-rescue operation was undertaken at the lake, it shifted one day later from a rescue mission to a recovery one, with Rivera presumed dead.
"Investigators believe Rivera drowned in what appears to be a tragic accident," the press release reads.
A day after Rivera's disappearance, Josey's father Dorsey, 36, was photographed leaving his home in Los Angeles. The Daily Mail then spotted Dorsey holding Josey. Earlier in the day, police said the boy "is in good health."
Speaking with PEOPLE about the incident, Ventura County Sheriff’s Office Sgt. Kevin Donoghue said that the "little boy was interviewed and based on the information he provided we are confident she is still in the water."
Jon Kopaloff/FilmMagic Ryan Dorsey and Naya Rivera
At this time, there is no evidence that Rivera made it to shore — leading authorities to believe she has drowned as emergency responders have searched the "shoreline for any clues whatsoever, any personal items, any footprints, anything that would indicate that she made it out of the water and because of the absence of those clues, that is why the focus of the search right now is in the water," Donoghue said.
"She could have suffered a medical episode. We just don’t know. We won’t know until we find her," Donoghue told PEOPLE.
On Wednesday, "the entire shoreline and surface of the lake were thoroughly searched. Divers also searched underwater in the area where the boat had last been seen. However, there was no sign of Rivera," the press release states.
Donoghue explained that Rivera's car was found in the parking area and her "personal belongings were on the boat."
"Her purse and her ID," he said.
Donoghue said a life jacket was also still on the boat when officials found her young son alone. However, he said: "We don't know if that just meant it was hers or if that was an extra life vest on the boat. But seemingly, if she was somehow incapacitated and she had a life jacket, we would find her floating. Clearly, she is not."
naya rivera/instagram Naya Rivera and her son, Josey, swimming on July 4, 2018
Police are still trying to figure out a timeline of when Rivera and her young child separated. "We don't know if the boy had been sleeping there for an hour or five minutes. She really could have been anywhere on a large portion of the lake," Donoghue shared.
During a press conference on Thursday afternoon addressing Rivera's disappearance, Donoghue said: "If the body is entangled in something underneath the water it may never come up."
Due to the conditions of Lake Piru, Donoghue said the search for Rivera is "complicated."
He explained that at this time, the visibility of the lake "is not very good, it's terrible."
"There's a lot of trees and plants and such that are under the water that can cause entanglement. Makes it unsafe for divers and a complicated search," he said.
Though the search for Rivera is continuing, "difficult conditions have made the recovery operation a very slow process," the press release states.
"Approximately 100 personnel are participating in the mission on Thursday, including resources from neighboring counties and the United States Coast Guard," the release reads.