Five ranch workers have been indicted in the slayings of more than 40 wild horses and burros in Nevada, and officials said the investigation continues into the deaths of 500 more animals.

The men were charged with violating the federal Wild and Free Roaming Horse and Burro Act, Assistant U.S. Attorney Will Mattly said Wednesday. Each count carries up to a year in prison and a $2,000 fine.Mattly said the slayings occurred on federal land 150 miles northeast of Reno where the remains of more than 500 mustangs have been found since August.

Many of the animals had been shot, although others may have died of natural causes, officials said. Some of the animals may have been dead for years.

Mattly gave no motive for the killings, and the charges, contained in an indictment handed up Tuesday, did not specify how the animals died.

Indicted were David Morehead and Shannon Brennan of Lovelock, cousins Eugene John Thacker of Winnemucca and David Thacker and Ronald Emory Hage of Cosgrave. They were taken into custody Tuesday and Wednesday.

David Thacker's father, John, said, "I don't believe any of them are guilty." Morehead's attorney, Don Hill, said his client is also innocent.

Maxine Shane, a spokeswoman with the U.S. Bureau of Land Management, said the agency considered the case to be far from over.