The National Weather Service says a tornado that struck a workers' camp in the heart of North Dakota's booming oil patch Monday night was an EF-2 with winds peaking at 120 mph.
The weather service says a second brief tornado possibly occurred in the area based on eyewitness accounts.
Nine people at the camp were injured, including a 15-year-old girl who remains hospitalized in an intensive-care unit with a head injury. The girl was visiting an aunt and uncle at the camp.
The twister damaged or destroyed 15 trailers at the camp just south of Watford City.
North Dakota's oil boom has brought thousands of people to the area looking for work. Many live in hastily assembled trailer parks, known as man camps, with prefabricated structures that resemble military barracks.