Twenty-seven horses have died in what appears to be an accidental poisoning at a Brazos County, Texas boarding barn, reports The Bryan-College Station Eagle, a local newspaper.

Preliminary reports suggest that the horses’ feed might have been contaminated with a highly toxic substance. Necropsies are being performed on the dead, while 18 others exhibiting similar clinical signs were under observation and treatment as of Wednesday night (July 19).

The Eagle reports that early testing by Texas A&M University (TAMU) in College Station has revealed phosphine gas in the stomachs of three of the dead horses. Phosphine gas is a byproduct of a weevil treatment farm owner Bradley Raphel used on the horses’ grain. The question that remains is why the gas, which normally dissipates after treatment, would remain in the feed. Raphel reported to the paper that he has used the same weevil treatment in the facility’s grain silo on numerous prior occasions with no problems.

Raphel’s wife, Beverly told the Eagle that two-thirds of the afflicted horses belonged to the family while the others were boarded animals