A necropsy on a horse that died after it was attacked by dogs in Waialua could not determine a cause of death and state officials will conduct further tissue studies, a state Department of Agriculture spokeswoman said Wednesday.

Janelle Saneishi, Agriculture Department spokeswoman, said the necropsy was done on the 3-year-old filly on Tuesday, but "there was no obvious cause of death."

She said state veterinarians were called to assist a private veterinarian try to find the cause of death and will do tissue tests to determine if there was a pre-existing condition that may have killed the horse.

Police said the incident happened about 9:20 p.m. Monday when a witness heard dogs barking and saw the dogs attacking a horse. When officers arrived, several dogs remained and were picked up by the Hawaiian Humane Society.