WINDSOR, Colo. — A large tornado caused heavy damage in two northern Colorado towns on Thursday, flipping over tractor-trailers and ripping roofs off buildings.

No serious injuries were immediately reported.

Television footage showed homes and buildings with roofs missing and farm irrigation equipment crumpled.

"We have a lot of damage at this point," said Brenda Stoman of the Windsor Fire Department. She said the tornado knocked down trees and shattered windows.

Buildings were also damaged in Greeley, about 10 miles east of Windsor, said Weld County sheriff's spokeswoman Margie Martinez. She said two tractor-trailer rigs were overturned.

Electricity was knocked out to about 20,000 customers in northern Colorado, Xcel Energy said.

Interstate 25 was closed in northern Colorado.