Police officers in the eastern Iowa town of Bellevue have concluded their investigation into the July 4th parade that turned deadly.

Bellevue police say it’s still unknown why two horses pulling a buggy bolted and trampled the crowd after the bridle came off one of the horses.

Sixty-year-old Janet Seines, a passenger in the buggy, was thrown off the buggy after it hit a tree and died from her injuries. The initial report said 24 other people were injured as the horses bolted through the crowd, but police have since identified five more victims to bring the total number injured to 30.

Police say they have ruled out several things that could have startled the horses — including the sound of fireworks, sirens, or children throwing candy. Officers cite witnesses who say birds in a nearby birdhouse were agitated by the parade and were swooping — but officers say there’s no evidence to conclude that’s what caused the horses to bolt.

The report also says one of the horses ended up with three breaks in its leg and had to be put down.