A student armed with a gun was holding 23 children and a teacher hostage in a high school classroom in the small Wisconsin town of Marinette, police said.
The student, who was not identified, entered the classroom at the end of the school day, local media reported.
"We believe there are 23 students in the classroom and the teacher," Marinette police chief Jeffrey Skorik told a news conference, adding, "We have no idea about the motivation."
"We want to encourage this young man ... to come out," he said.
Skorik said hostage negotiators were at the scene, and police were interviewing the suspect's parents about potential motives.
As far as authorities could determine, he said, the gunman was acting alone and was restricted to one location.
"We have no reason to believe that we have more than one actor," he said.
Skorik said reports that some people had been injured in the hostage-taking were wrong.
"There have been some serious injuries reported on Facebook that we know are not true," Skorik told reporters.
Marinette is north of the city of Green Bay, near Michigan's upper peninsula.
Police closed off roads around the school.
Parents awaiting children they believed to be inside were asked to gather in a nearby court building, Kathy Frank, a spokeswoman for the county emergency management office said.