SEARSPORT — The deaths of seven horses in Searsport point toward an accidental poisoning, according to the state veterinarian.

Veterinarian Don Hoenig said tests performed on the dead horses' manure validated the presence of an anti-bacterial additive in cattle feed. The problem is that the substance -- called monensin -- is deadly when given to horses.

Hoenig said the case represents the first of its kind in his 21 years in Maine.

He said he still doesn't know how the horses ingested the cattle feed. He said his tests reveal no contamination of the horses' feed at the farm in Searsport.