Germany´s n-tv reported that police said the suspect was dressed in a black combat uniform.
Regional police spokesman Klaus Hinderer said students were among the 10 killed, but he had no further details.
The shooter entered Albertville high school in Winnenden at 9:30 a.m. and opened fire, shooting at random, before fleeing, police said.
Police warned area residents not to pick up anyone in their cars as they searched for the suspect, described by Germany´s N24 television as a 17-year-old former student who was known to police.
Hinderer could not confirm media reports that the shooter was a former student at the school.
Hinderer said the suspect fled toward the center of Winnenden, a town of 28,000, Hinderer said. The school was evacuated.
About 1,000 children attend the school, located in a suburb some 12 miles (20 kilometers) northeast of Stuttgart.
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