(CNN) -- A fast-developing storm spawned at least one tornado Sunday in Billings, Montana, ripping the roof off a massive entertainment complex and causing extensive damage in the region.
No injuries were reported.
Amateur video shows the funnel cloud hovering over the MetraPark venue -- home to a sports arena, expo center and other facilities -- kicking up debris and peeling away the roof of one of the complex's buildings.
Gov. Brian Schweitzer estimated that the roof damage could cost between $8 million and $15 million, but noted it was fortunate the arena was not hosting an event when the twister struck around 4:30 p.m. (6:30 p.m. ET).
The damaged building held a football game at the same time just a night earlier, he said.
Meteorologist Keith Meier of the National Weather Service in Billings estimated the tornado to be as strong as an EF2 on the Enhanced Fujita scale, with winds estimated at more than 100 mph.
Meier said the storm also produced quarter- to half-dollar-sized hail.
Schweitzer plans to travel to the region Monday to survey the damage; he said the National Guard is ready to assist in cleanup efforts if needed.
CNN's Jessica Jordan and Justin Lear contributed to this report.