WEST PALM BEACH, Fla. — Florida officials say a mineral overdose is the probable cause of death for 21 polo horses that fell ill as they prepared for a championship match earlier this month.

The Florida Department of Agriculture and Consumer Services said Tuesday the animals likely overdosed on selenium, a common mineral that can be toxic in high doses.

The horses from the Venezuelan-owned Lechuza Caracas team began collapsing as they were unloaded from trailers at the International Polo Club Palm Beach in Wellington. Some died at the scene, others hours later. All 21 horses were dead by the next day.

The toxicology results say the animals had "significantly increased selenium levels."

Necropsies previously revealed bleeding in the horses' lungs.