Police searching for missing Naya Rivera fear her body has become "entangled underwater" and may never be found.

The Glee star, 33, disappeared from a boat on Lake Piru in California on Wednesday and local authorities subsequently launched an extensive search operation.

The alarm was raised after her son Josey, four, was found alone on the boat hours later. She remains missing and presumed dead.

The search has been underway since Wednesday, but despite using specialist sonar equipment, trained dogs and divers, Ventura County Sheriff's Office sergeant Kevin Donoghue fears the star's remains could be stuck underwater.

Speaking to the PA on Friday, he explained: "If the body is entangled in something beneath the water, it may never come back up. We don’t know."

Glee actress Naya Rivera has been presumed drowned after going missing in a lake

Officials at the Ventura County Sheriff's Office have also revealed they been battling poor visibility as the underwater search continued.

In a post on the force's official Twitter page, they shared footage from one of the cameras they had been using showing how murky the water was at a depth of 30 feet.

The lake is nearly two miles long and 130-feet deep at some points.

The search for the star has been going on since Wednesday

Donoghue also explained he believes the TV star's remains will be in the water as there is no evidence to suggest she made it to shore.

He told the PA: "We searched with people on the ground, on the shoreline.

"We were looking not just for her physically, we were looking for any clues, any evidence that suggested she made it to shore.

"We didn't find any clues, any evidence that suggested she made it to shore.

"The most conclusive piece of evidence we're really holding on to right now is the statements from her son, the only witness.

"He observed his mother disappear beneath the water. So we are very confident she is in the water."

He added that the police force remains hopeful that "at some point in time we will recover her from the lake."

Josey was unharmed and is now being cared for by his dad, Naya's ex Ryan Dorsey, 36, and his grandparents.