"It sounded like all the doors were being torn off the house," said Kelly Keil, who grabbed her 5-year-old daughter and took cover in a closet in her home, which was spared major damage.
Gov. Bill Ritter toured the area and declared a local state of emergency, but an inventory of damaged homes had to wait until daylight Friday. Federal, state and local officials were assembling damage assessment teams overnight.
Severe storms, some including tornadoes, also ripped through parts of Wyoming, Kansas and California on Thursday.
Heavy equipment cleared trees, utility poles, and mangled wood and metal from the streets of the east Windsor neighborhood where the most damage occurred. Police enforced an overnight curfew to deter looting and ensure residents' safety in case of natural gas leaks, while officers with search dogs went door to door to look for anyone missing.
Resident Loree Wilkinson, 39, and her children, ages 6 and 9, huddled in a basement and prayed as the tornado passed overhead. She said her youngest child, Kazden, prayed: "Please don't let me die because I just graduated from kindergarten."
The large storm cloud descended nearly without warning, touching down near Platteville, about 50 miles north of Denver. Over the next hour, it moved northwest past several towns along a 35-mile-long track and into Wyoming.
Oscar Michael Manchester, 52, was killed at a campground west of Greeley, about 60 miles north of Denver, said Weld County Deputy Coroner Chris Robillard. Pete Ambrose, caretaker at the Missile Park campground, said Manchester was in a recreational vehicle that was destroyed by the storm.
Nine people were hospitalized with various injuries at the Medical Center of the Rockies in Loveland, spokesman Alex Stuessie said. In Greeley, four people were treated for minor injuries at North Colorado Medical Center, administrative representative Laurie Hamit said.
The Red Cross served food to about 130 people in Windsor who were displaced by the storm, but by nightfall only one family was staying at a shelter at a fairgrounds outside town.
The tornado overturned 15 railroad cars and destroyed a lumber car on the Great Western Railway of Colorado, said Mike Ogburn, managing director of Denver-based Omnitrax Inc., which manages the railroad. Fourteen of the overturned cars were tankers, but they were empty.
The twister toppled tractor-trailers across Highway 85 and cut power to 60,000 customers. Electricity was restored to all but 15,000 by late Thursday. Xcel Energy said it lost two large transmission lines and about 200 utility poles. The utility also responded to a handful of natural gas leaks at homes that were ripped off their foundations, spokesman Tom Henley said.
Jim Kalina, a National Weather Service meteorologist, said two or three major storm cells affected the area and officials were trying to confirm how many tornados touched down.
Weld County is known as a prolific tornado spawning ground, with about seven typically reported there each year, according to the weather service.
In Kansas, early reports indicated that about 10 tornadoes passed through the western part of the state Thursday evening, said Scott Mentzer, a weather service meteorologist in Goodland.
He said a few barely touched down, but a couple moved along 30 to 50 miles on the ground in Sheridan and Decatur counties. Authorities said the tornadoes destroyed one home and damaged several others.
Officials were trying to verify whether a tornado touched down in Laramie, Wyo., where a storm packing strong winds damaged several buildings, overturned vehicles and knocked out power Thursday afternoon.
Later, a tornado touched down in a rural area near the town of Burns, Wyo., about 10 miles east of Cheyenne, said Rob Cleveland, director of Laramie County Emergency Management. The storm did minor damage to two homes and destroyed a barn, but there were no injuries, Cleveland said.
Elsewhere, a storm system that lashed Southern California on Thursday unleashed mudslides in wildfire-scarred canyons, spawned at least two tornados and dusted mountains and even low-lying communities with snow and hail.
Powerful wind or a funnel cloud toppled a tractor-trailer and freight cars, said Riverside County fire spokeswoman Jody Hageman. California Highway Patrol Officer Alex Santos was watching the wild weather from a highway overpass in Moreno Valley, about 60 miles east of Los Angeles, when he saw two tornados closing in.
"There was so much dust you couldn't see. Next thing I know I see this big rig getting toppled over," Santos said. He said the driver had to be cut free from the cab and suffered head and back injuries.
About 100 people have died in U.S. twisters so far this year, the worst toll in a decade, according to the weather service, and the danger has not passed yet. Tornado season typically peaks in the spring and early summer, then again in the late fall.