NORMAN, Okla. — At least one of the massive tornadoes that killed hundreds across the South was a devastating EF-5 storm, and the National Weather Service expects to determine later today that “many more” also were the worst of the worst.

After the first day of assessing storm damage, the weather service said the tornado that hit Smithville, Miss., at 3:44 p.m. EDT on Wednesday was an EF-5 storm. That’s the highest rating given to assess a tornado’s wind speed, and is based in part on damage caused by the storm.

The weather service said the half-mile wide Smithville tornado had peak winds of 205 mph and was on the ground for close to three miles, killing 14 and injuring 40.

It was the first EF-5 tornado to strike Mississippi since 1966, and the first EF-5 tornado in the United States since a May 25, 2008, storm in Parkersburg, Iowa.

Meteorologist Jim LaDue at the weather service’s Storm Prediction Center in Norman, Okla., said he expects “many more” of the tornadoes that killed at least 297 people during Wednesday’s brutal outbreak of severe weather will be rated EF-5 later Friday.

The weather service also is sending assessment teams back out to review more damage in several states, including Mississippi and Alabama. The assessments are preliminary, and are based on photos taken during the ground surveys and consultation with experts. They will be confirmed later this year.

The Smithville tornado rated EF-5 destroyed 18 homes, which the weather service said were well built, less than 10 years old and bolted to their foundation.

The plumbing systems and appliances in the worst part of the storm’s damage path were either shredded or missing entirely, as is a 1965 Chevrolet pickup truck that was parked in front of one of the destroyed homes.