Ten tornadoes touched down in several small communities in North Texas overnight, leaving at least six people dead, dozens injured and hundreds homeless.
Emergency responders were still searching for missing people.
The National Weather Service gave a preliminary estimate of Wednesday night's violent system, saying a tornado in Granbury had wind speeds between 166 mph and 200 mph.
Other tornadoes damaged nearby Cleburne and Millsap.
Granbury, southwest of Fort Worth, saw the worst of the damage, as the exceptionally powerful tornado tore through two neighborhoods around 8pm.
Resident Elizabeth Tovar said fist-sized hail heralded the tornado's arrival and prompted her and her family to hide in their bathroom.
"We were all, like, hugging in the bathtub and that's when it started happening. I heard glass shattering and I knew my house was going," Tovar said, shaking her head.
"We looked up and ... the whole ceiling was gone."
The NWS' preliminary storm estimate was an EF-4, based on the Fujita tornado damage scale. An EF-5 is the most severe. The powerful storm crushed buildings into piles of planks and rubble. Trees and debris were scattered across yards, and fences were flattened.
Hood County Sheriff Roger Deeds described the overnight hunt for bodies in Granbury.
"Some were found in houses. Some were found around houses," Deeds said. "There was a report that two of these people that they found were not even near their homes. So we're going to have to search the area out there."
Seven people remain unaccounted and authorities hope they are with family or friends, Deeds said at a news conference. Emergency responders were working to identify the six adult bodies that had been found, he said.
He said 37 injured people were treated at hospitals.
One of the neighbourhoods, Rancho Brazos, had a significant number of Habitat for Humanity homes that were damaged.
Harold Brooks, a meteorologist at the NWS' severe storm lab in Norman said May 15 is the latest into the month that the US has had to wait for its first significant tornadoes of the year.
Brooks said he would expect 2013 to be one of the least lethal tornado years since the agency started keeping records in 1954.
Utilities said about 20,000 homes and businesses in the region were without power early Thursday.
Another tornado cut a mile-wide path through Cleburne, storm spotters told the National Weather Service.
Cleburne Mayor Scott Cain said Thursday morning that no one was killed or seriously hurt in the courthouse city of about 30,000. Seven people suffered minor injuries, and upward of 150 homes were damaged and another 50 were destroyed.
He described the storm as "bizarre" because severe winds kept shifting in different directions.
Cleburne resident Derrek Grisham was going through his mother's damaged home, salvaging items before the home is likely torn down.
Grisham, 26, said after the storm passed through, he ran to his mother's nearby home to check on her and his 10-year-old son, who was staying with her.
"I had to kick in the front door to get them out," he said, explaining the two had taken shelter in a bathtub.
The roof of the home was torn away and he said her belongings were a jumbled mass, but that Catholic crucifixes stayed in place on the living room wall.
Another tornado hit the small town of Millsap, about 40 miles west of Fort Worth. Parker County Judge Mark Kelley said roof damage was reported to several houses and a barn was destroyed, but no injuries were reported.