Eight other people were treated at a Watford City hospital for less serious injuries. The American Red Cross said eight residents spent the night at a shelter at Watford City's Civic Center and that several families were among those displaced.
It was cool and rainy Tuesday morning at the site where the tornado hit, and there was little activity.
Plywood was scattered along with other debris across several hundred square feet at the site. Four trailers and a couple of other prefabricated buildings were still standing.
A heavily damaged truck was flipped over on the highway and several other abandoned vehicles were nearby. Road signs were flattened and tumbleweeds pushed up against some electrical wires.
Don Dailey, who lives in a camp about 200 yards from the one that was hit, said workers got a weather service tornado alert on their cellphones about the same time they saw the funnel coming down to the ground. He and others took cover behind a large piece of excavating equipment.
Samuelson said all those injured had been inside their trailers when the tornado struck.
National Weather Service meteorologist Todd Hamilton said two meteorologists and an emergency response specialist left Bismarck at daybreak Tuesday to survey the damage at the camp.
In a statement Tuesday afternoon, the weather service said preliminary information suggests the winds of the EF-2 twister peaked at 120 mph. The weather service said a second brief tornado possibly occurred in the area based on eyewitness accounts.
The oil boom has brought tens of thousands of people into the area looking for work. Many live in hastily assembled trailer parks, known as man camps, housing prefabricated structures that resemble military barracks. Some companies rent blocks of hotel rooms for employees, and some workers sleep in their cars or in tents.
The camp hit by the tornado was relatively small. Samuelson said some camps in McKenzie County have hundreds of trailers and that the possibility of injuries or damage would have been much higher if one of those had been hit.
It was not immediately known who owned the camp that was hit.
Target Logistics is the largest crew camp operator in the oil patch, with more than 5,000 workers in nine facilities. Company regional vice president Travis Kelley said a weather radio is monitored by staff at each facility. If a tornado is reported in the area, workers are "encouraged to come to common areas such as recreation or dining areas, which are fairly well protected right in the middle of the facility," he said.
Tornadoes are rarely reported in McKenzie County, with only 14 since 1950, with no fatalities, according to weather service data.
"If the man camps weren't there, this tornado might never have been reported," meteorologist Ken Simosko said.
The growth of temporary housing also means there is more of a chance for death, injury and destruction from tornadoes, he said.
"People living in trailers creates a very dangerous situation because there is no protection," Simosko said.
Dan Yorgason, who lives in a neighboring camp, filmed the tornado from inside his truck.
"There was literally nowhere to go," he said. "The tornado was coming down the hill along our only escape route. There was nowhere for us to go. It was crazy."
Associated Press writers Blake Nicholson, James MacPherson and Kevin Burbach in Bismarck; and Carson Walker in Sioux Falls, South Dakota, contributed to this report.
Contact Wood at https://twitter.com/JWoodAP .