The CNN quoted St. Mary's County Sheriff Tim Cameron as saying that the school resource officer engaged the shooter and ended the threat, which occurred in a hallway just before classes began.
The Sheriff said that the gunman was later pronounced dead.
Earlier, St. Mary's County public information office Andrew Ponti had said that at least three people were injured in the shooting incident.
"It's unclear if the shooter was among the injured," he had said.
The injured were rushed to a nearby hospital for medical assistance.
Student at Great Mills Jonathan Freese, who was lockdown in math class, called CNN from his cell phone and said Police were checking all the classrooms.
"I'm still a little shaken up," he said. "I didn't really expect for this to happen. I do always feel safe, though, because they always have police at the school."
Earlier, the St. Mary's County Sheriff tweeted, "there has been an incident at Great Mills High School. Parents please DO NOT respond to the school."
Earlier this week, thousands of students across the US walked out of their schools to call for action against gun violence precisely one month post the Florida shooting which claimed 17 lives. (ANI)