Underwater Footage from Naya Rivera Search Released: She Could Be Found in '5 Minutes' or '5 Days'

The search for Naya Rivera at Lake Piru continues, two days after she disappeared during a boating trip with her son.

Authorities searching for the missing Glee star released footage late Friday to show just what they are up against as they comb the lake's floor.

Ventura County Sheriff's Office released video obtained when they deployed a remotely operated vehicle (ROV) 30 feet from Lake Priu's surface during the search for the star's body.

The ROV was sent down to investigate a dark shape which turned out to be a tree branch. The footage shows just how low the visibility is.

During a press conference on Friday, authorities said that while sonar equipment has picked up several underwater images, none of the objects detected are Rivera.

"Unfortunately, they did not locate Naya Rivera. They are still searching," Ventura County Sheriff's Office Captain Eric Buschow told reporters, calling the situation a "difficult" one because "we don’t know if she’s going to be found five minutes from now or five days from now."

Lake Piru, which is located about 57 miles north of Los Angeles in Ventura County, California, is nearly two miles long and 130-feet deep at some points.

Buschow explained that responders are focusing on the north and eastern sides of the lake, and are using a side-scanning sonar to help focus the divers' search.

Ventura County Sheriff Sgt. Kevin Donoghue tells PEOPLE that the sonar device "paints pictures of the lake bed floor, and when it sees an anomaly, an object that's consistent with the shape and size of what we're looking for, then we send divers to search that specific area."

Given the size of the search area, this makes for a much more efficient and targeted investigation.

Donoghue tells PEOPLE that there are currently three sonar devices out on the water, and Buschow said that there are about 40 personnel involved in the search on Friday.

"We can’t predict" when she will be found, Buschow said at the press conference. "We may find her in an hour from now or several days from now. So there’s no way to predict that."

Rivera, 33, was presumed dead on Thursday, one day after her 4-year-old son Josey was found alone on a pontoon boat at Lake Piru, where they had been swimming.

Josey was deemed "in good health" and has been reunited with his dad and Rivera's ex-husband, Ryan Dorsey. The toddler told investigators that his mom never got back into the boat after they had been in the water, according to a press release from the Ventura County Sheriff's Office released Thursday.

Donoghue tells PEOPLE that while the events that led to Rivera's disappearance are "still a mystery," he adds that there is no reason to suspect foul play or that Rivera was attempting suicide.

"I think at some point in time we will find her," Donoghue says. "I will say that we are hopeful to find her. We want to bring peace and closure to the Rivera family. This is a tragic event."

"If we don't find any clues today, we'll still be out here tomorrow," he says.

As the search for the Glee star continued on Friday, poor visibility conditions at the lake made it difficult.

"The visibility is about one to two feet," volunteer diver Max O'Brien told reporters Friday. "... 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 said. "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."

Donoghue said on Thursday that responders "don't know" if Rivera's body will be able to be recovered. "If the body is entangled in something underneath the water, it may never come up," he said.

Also on Thursday, the sheriff's office released CCTV footage of the lake's dock, which showed Rivera and Josey arriving, renting a boat, and launching northwards on the lake. The mother-son pair were alone, each with a backpack, and the actress also carried a large white sack.

Lake Piru is known for its dangerous conditions, and Donoghue previously confirmed to PEOPLE that there have been prior drownings on the lake. A Change.org petition has been created following Rivera's disappearance, urging officials to post warning signs for those interested in swimming.

The lake will remain closed to the public as long as the search for Rivera continues, Buschow said.