(CNN) -- More than 20 horses became ill when they were intentionally poisoned with toxic leaves in southern California this week, authorities said Friday.

These oleander bush leaves, toxic to horses, were found in a San Diego, California, stable.

The horses were given oleander bush leaves between Wednesday night and early Thursday morning at their ranch in San Diego, the county sheriff's department said in a news release.

The leaves were found in the stalls in the morning by employees arriving to work, according to officials.

"We found bits and pieces of carrots and chopped up apples along with multiple leaves," ranch owner Bill Tomin told CNN television affiliate KFMB.

"This is horrible, it's scary why would someone do such a thing," he said. "If they were angry with me, burn my house down, slash my tires, confront me."

An exact number of horses sickened was unclear, but officials believe they were more than 20. The animals, whose prognosis was not known, were under the care of veterinarians.

An investigation is under way.

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