Law enforcement estimated 30 people remained in the evacuation area in the morning, but one observer put the number higher.

About a dozen people have crossed the frozen Cannonball River and re-entered the camp as of 9 a.m. Thursday. Cleanup was scheduled to begin at 9 a.m.

"We don't know exactly what's inside some of the remaining structures," Iverson said before officers moved into the camp. A total of 11 people were arrested Wednesday, according to a list of names provided by the Morton County Sheriff's Department.

The U.S. Army Corps of Engineers previously announced it would close the camp Wednesday, and Gov. Doug Burgum signed an evacuation order with a Wednesday deadline, emphasizing safety concerns and the need to clean up the camp ahead of spring flooding. Burgum said Wednesday evening that people could still leave voluntarily Thursday.

Those arrested Wednesday included one member of the media, Jack Smith IV of Mic, an online news outlet. Iverson said everyone was warned to leave Highway 1806 or face arrest.