Divers Search by Touch Underwater for Naya Rivera Due to Low Visibility: 'Kind of a Braille Search'

"Under the water, it's a lot by feel," one diver explained to reporters of the difficulty of the recovery mission

Authorities are still searching for Naya Rivera after she disappeared during a boating outing with her 4-year-old son, who was found safe on Wednesday.

Ending a day of searching on Thursday, local officials and search volunteers explained their process for the mission as they wait for daylight to resume. One volunteer diver, Max O'Brien, told reporters that the combing of Lake Piru's bottom is done mostly by touch since the visibility is so low.

"The visibility is about 1 to 2 feet," said O'Brien, "... There's a lot of tree branches and overgrowth from when the lake was lower, so we're digging through, breaking through sticks and searching a heavy brush bottom."

"Under the water, it's a lot by feel," he explained. "Again, there's a lot of shrubbery and sticks that we have to break through as we're going through, so it's kind of a Braille search."

Authorities described the operation as a "very slow process" due to the conditions of the lake. Approximately 100 people participated in the search on Thursday.

The same day, authorities shifted the mission from a rescue to a recovery as the Glee actress, 33, has been presumed dead. According to the Ventura County Sheriff's Office, Rivera rented a boat at the California lake on Wednesday afternoon while with her son Josey.

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Their boat was then found drifting in the northern portion of the lake with a sleeping Josey by himself onboard. The boy told investigators that he and his mom went swimming, but that Rivera never returned to the vessel.

During a press conference on Thursday, Ventura County Sheriff’s Office Sgt. Kevin Donoghue said: "If the body is entangled in something underneath the water it may never come up."

Officials added that they "don't know" if the actress' body will be recovered. Donoghue echoed that the conditions of Lake Piru complicate the endeavor, explaining that visibility 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.

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Ahead of the press conference, police told PEOPLE that they are "confident" Rivera is still in the water.

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.