"On this day, we realise our worst nightmare," Cameron said, "that our greatest asset, our children, were attacked in one of our places, a bastion of safety and security, one of our schools."

"The notion of it can't happen here is no longer a notion," Cameron added.

The shooting comes just over a month after 14 students and three adults were killed at a high school in Parkland, Florida, which has added new urgency to the nation's debate over gun control and raised questions about the role of armed personnel in schools.

Surveillance video taken during the shooting in Parkland showed Scot Peterson, the sheriff's deputy posted at the school, did not go inside a building to engage the gunman, in an apparent violation of protocol.

Speaking at a late morning news conference, Cameron said the school resource officer and witnesses to the shooting were being interviewed by detectives. Other students at the school had been taken to Leonardtown High School, about 15 minutes away, to be reunited with their families.

The parking lot at Great Mills High was filled with more than a dozen police and state trooper cars, the red and blue flashing lights punctuating the gray skies. Officers, some in neon yellow rain jackets, directed traffic in the pouring rain, having blocked off the area in front of the school with cars and caution tape.

"What we know thus far is that Great Mills High School began its school day this morning at 7:45am like many other days," Cameron said, "except for on this day a male student produced a handgun and fired a round wounding a female student and another male student in a hallway of Great Mills High School just before classes begun."

In a statement, the school described the shooting as "tragic" and urged parents and students to stay away as the school was locked down.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives sent agents to the school.

Governor Larry Hogan said his office was closely monitoring the situation, and that he was praying for the victims and the Great Mills community.

"But prayers are not enough," Hogan said, in an emailed statement. "Although our pain remains fresh and the facts remain uncertain, today's horrible events should not be an excuse to pause our conversation about school safety. Instead, it must serve as a call to action."

Reuters