SAN JUAN, Puerto Rico — Nearly 20 race horses, packed inside a double-decker trailer, were forced into crouching positions during a four-day sea voyage from the U.S. to Puerto Rico _ causing injuries that have prompted a federal animal abuse investigation.

Many of the thoroughbreds were bruised or bloodied when they arrived last month from Jacksonville, Florida, and four were so frantic veterinarians say they had to sedate them to get them out of the metal trailer.

The shipment, which is now under investigation by the U.S. Agriculture Department, involved a trailer designed for animals with shorter necks such as cattle. Animal rights advocates have sought to ban the trailers for the transport of horses.

"What's horrible about this is the way they were packed into that double-decker without any headroom," said Jose Garcia, chief veterinarian at the thoroughbred racetrack in the north-coast city of Canovanas, where the trailer arrived May 12.

"If you're going to stick horses on a boat and keep them there for four days, they've got to have room to move," said the veterinarian, who filed a complaint with police.

Puerto Rican police tracked the shipment to a Miami-based export company, Optical People Inc., before handing the investigation off to federal authorities. The Agriculture Department is investigating, said Animal and Plant Health Inspection Service spokeswoman Madelaine Fletcher.

Investigators could refer the case to the Justice Department to pursue criminal charges if they find evidence of abuse.

The president of the island horse owners association, Hector Gonzalez, and an attorney for the export company, Hiram Pagani, said the complaint is groundless and that none of the owners took issue with the horses' health upon delivery. Owners of the horses could not be reached independently.

The president of Optical People, Manolo Gonzalez, declined to comment.

The U.S. Caribbean territory imports hundreds of thoroughbreds from the U.S. mainland each year for racing, mostly by airplane.

The transportation by boat was promoted as a cheaper option by Hector Gonzalez of the horse owners association.

In March, he sent a memo to owners that said the Florida company would charge them US$750 per horse _ about half the cost of airfare. The memo, obtained by The Associated Press, described the sea-bound trailer as a "more economical and safe alternative."

The shipment's double-decker cattle trailers are notorious among animal rights activists who complain they force horses to stand in painful, crouched positions. U.S. federal law prohibits using the trailers to bring horses to slaughterhouses, but animal welfare advocates want a wider ban.

"It is bad enough on a road surface, but considering the turbulence on an open ocean, it boggles my mind," said Keith Dane of the Humane Society of the United States.

Some of the 19 horses initially refused to leave the trailer when it arrived at the track.

Staff members from Garcia's veterinary clinic were called to coax them out and documented injuries with photographs and a video camera. One horse had a deep gash along the top of its head. Another was still crouching five days later, Garcia said.

The police officer who led a Puerto Rican investigation, Maria Romero, said the same trailer apparently has been used for up to five horse shipments.

Romero said the case was referred to U.S. investigators because the alleged negligence began in Florida.