The free-roaming mustangs were shot with high-powered rifles at close range. Some of the wounded suffered for many hours before they were discovered and put out of their misery. At least one horse was blasted with a chemical fire extinguisher, officials say.
Investigators vowed to find the killers. Late Tuesday, they announced they have three suspects in custody.
"In this case it was as a result of very good detective work - painstaking, grinding work," said Washoe County Sheriff Richard Kirkland.
"We had one lead that was a little better than the others that actually started with an employee of the sheriff's office's granddaughter who was at a party who heard a certain thing being said," Kirkland said.
Anthony Merlino, 21, a Reno-area construction worker, was held on charges of grand larceny, grand theft and maiming, poisoning or killing another person's animal.
Arrest warrants on the same charges -- which carry combined maximum penalties of more than 10 years in prison -- have been issued for two Marines being held at bases in California.
Lance Cpl. Scott Brendle, 22, was being held at Camp Pendleton, Calif., and Lance Cpl. Darien Brock, 21, was being held at Twentynine Palms east of Los Angeles, Storey County District Attorney Janet Hess said Tuesday night.
"It's sad but true," said Major Lenny Ryan of Camp Pendleton.
Both are members of the 1st Marine Division. Both were residents of the Reno area home on holiday leave when the horses were shot Dec. 27.
Hess said it was not clear if some form of extradition would be necessary, but that prosecutors intended to return the soldiers to Nevada to face the charges.
Law officers said they did not believe military weapons were used in the shooting.
Animal protection groups raised $35,000 in reward money. The crime touched people far away from the Nevada desert.
"We've had citizens from across the country offer us anything from psychic tips to offers to come out and bury the horses," said Sheriff Pat Whitten of Story County.
Although ranchers often complain about overgrazing by wild horses, that dispute had nothing to do with the killings, investigators say. They have not explained why three young men may have slaughtered these symbols of the American West.