In a statement Tuesday, Florida Agriculture Commissioner Adam Putnam said a preliminary investigation determined that excessive speed caused the Sand Blaster Roller Coaster at the Daytona Beach Boardwalk to derail.
Putnam said subpoenas were issued to the ride operator so his agency can complete its investigation.
The ride has been shut down since the derailment. Putnam said it will remain closed indefinitely.
Nine people were injured, including two who fell to the ground, after the ride's front car came completely off the tracks.
The ride had been shut down by state inspectors twice in the past 18 months because of problems.