The shooting took place at Great Mills High School in St Mary's County, and it was not clear whether there were injuries.
A Maryland high school was on lockdown following a shooting on Tuesday morning, the local school district said in a statement, but the incident has since been "contained."
The shooting took place at Great Mills High School in St Mary's County, and it was not clear whether there were injuries.
It occurred amid a re-energized national debate over school shootings in the United States following an attack on Feb. 14 at Marjory Stoneman Douglas High School in Parkland, Florida, where a gunman killed 17 students and faculty.
The St. Mary's County sheriff's office confirmed an incident at the school and urged parents in a Twitter post not to approach the school. Federal investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were heading to the school, the agency said.
St Mary's County Public Schools said on its website that the school was on lockdown and the incident had been "contained" but provided no further details.
It said law enforcement was on the scene.
"It happened really quickly, right after school started" after 8:00 am (1200 GMT), Jonathan Freese, a student at the school, told CNN.
"The police came and responded really quickly," Freese said. "They had a lot of officers respond." "Right now, the police are going through classrooms," he said.
"Soon we are going to be escorted from the school." The Great Mills incident comes about five weeks after a shooting at a Florida high school left 14 students and three adult staff members dead.
Great Mills is a town about 70 miles (110 km) south of Washington.
In February this year, a former student armed with a powerful assault rifle unleashed a hail of gunfire in a Florida high school, killing 17 people and injuring 15 others, including an Indian-American student, in one of the grizzliest mass shootings in modern US history.
(With Reuters and PTI inputs)
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