Park staff have recovered several items that evidently were deposited in the park by the remnants of an EF5 tornado â the most severe type of tornado â that occurred approximately 100 miles southwest of Effigy Mounds. The tornado, which killed eight people, injured 70, and destroyed between 100 and 200 homes, occurred in Parkersburg, Iowa, on Sunday, May 25th. The National Weather Service has estimated that the tornado had wind speeds around 205 mph, making it the first EF5 tornado in Iowa since June 13, 1976. Recovered items consisted of personal photographs and what appeared to be legal documents. Park law enforcement is working with the Butler County Sheriff's Office to identify the affected parties and return the materials.