MASON CITY, Iowa (AP) – Federal investigators have released more information about a north Iowa helicopter crash that killed three medical workers. KIMT-TV says that a National Transportation Safety Board report details circumstances of the January 2013 crash near Ventura. The Mercy Air Med helicopter carrying a pilot, nurse and paramedic was on its way to pick up a patient in Emmetsburg when it crashed into a farm field.

Investigators focused on weather conditions at the time of the crash. The report says witnesses and first responders reported mist, drizzle, freezing precipitation and icy roads. Investigators said the helicopter wasn’t certified for such conditions.

Investigators will use the report to determine a probable cause of the crash.