A pair of animal-rights groups are suing the Union County Sheriff's Office over its use of a gas chamber to euthanize some animals at the county animal shelter. The lawsuit claims the sheriff's office violates N.C. law against cruel treatment of animals by using carbon monoxide gas to kill sick, injured, very old, very young or pregnant animals brought to the shelter in Monroe. Sheriff Eddie Cathey denied his office, which oversees the shelter, has done anything wrong. "The things we are accused of, the violations of statutes, (are) totally untrue," he said. He declined to elaborate, saying he had not been served with the complaint yet. The lawsuit was filed Friday in Union County District Court. The plaintiffs, the N.C. Coalition for Humane Euthanasia and the Humane Society of Union County, say using a gas chamber violates N.C.