Rescue workers struggled Thursday to save a herd of 100 horses stranded for days on a tiny knoll after a fierce storm turned their pasture into an angry sea.

Eighteen horses have drowned and the rest have spent two nights huddled together in knee-deep water. Rescuers planned to bring in horses that are strong swimmers to show the panicky animals, including several foals, how to get to dry land.

Dutch television and newspapers carried dramatic photographs and footage of the horses crowded together, their backs to the wind, on a small patch of ground. They were surrounded by brackish-colored sea water, pushed by the storm surge into a wilderness area outside the dikes of Marrum, 90 miles northeast of Amsterdam.

Marrum's fire department used small boats to ferry about 20 horses, including the smallest foals, to safety on Wednesday.

The Dutch army tried to rescue the rest of the herd, but called off the operation when water levels receded to less than three feet in some places, grounding pontoon boats.

Mayor Wil van der Berg said he considered using helicopters to transport the animals, but decided the noise and lights might panic the horses and cause more to drown.

Although horses can swim and the closest dry land was only several hundred yards away, there were concerns the animals could become ensnared in submerged barbed wire fences or that they might tire if they headed toward land farther away.

Water levels continued to fall Thursday, and the surviving animals were brought feed and blankets by boat. Veterinarians examined them, the Netherlands' state broadcaster NOS reported.

Since the remaining horses were in good health, officials decided to leave then on the knoll overnight. Other horses that are strong swimmers were to be brought in Friday to try to lead them to safety.

"Definitely don't go there at night. A few animals are in the water, but only up to their ankles and they can hold out another 24 hours," veterinarian Marten de Vries was quoted as saying in the Friesch Dagblad daily after he examined the horses. "There are still a few foals among them, but they're dry."

In a worst-case scenario, the horses could be stranded for several more days, NOS reported.

The Netherlands's Party for the Animals said it had filed a complaint against the horses' owner and the operator of the wilderness area, since the national weather service had put the country on alert for rising flood waters early Tuesday.

"Despite a number of worried warnings from neighbors, they did not act and are therefore in any case morally and legally responsible for the death of at least 18 horses," the party said in a statement. "We want to know what measures will be taken to prevent this kind of situation in the future."

The Agriculture Ministry ordered an inquiry. "We're going to work together with prosecutors to see whether there was any criminal act committed," said spokeswoman Anita Douven. "That could be negligence, or possibly mishandling of the animals."

She said the investigation would look at the role of both the owner, Pieter Lootsma, and the land operator, It Fryske Gea, which had ordered the animals cleared from the area for the winter by Oct. 15.

Lootsma told local media the animals had initially been brought to higher ground, but panicked Tuesday night when other herders and emergency service workers shined spotlights on them.

"In the end, they ran away from their safe place," Dutch press agency ANP quoted Lootsma's distressed wife as saying.