The National Weather Service says the tornado that killed at least 116 people in Joplin, Mo., appears to have had more than one vortex.
Storm Prediction Center director Russell Schneider says video evidence shows Sunday's tornado appeared to be a rare "multivortex" twister.
Multivortex tornadoes contain two or more small and intense subvortices that orbit the center of the larger tornado circulation.
Schneider says multivortex tornadoes are relatively uncommon but have been seen in massive storms like the one that ripped through Joplin.
The tornado was given a preliminary EF4 rating _ the second-highest rating given to twisters. The rating is assigned to storms based on damage they cause.
Sunday's tornado was the deadliest single U.S. twister in about six decades.