The female victim has been identified as 16-year-old Jaelynn Willey.
In 2001, an officer shot and wounded an 18-year-old student who wounded two teachers and three other students with a shotgun at Granite Hills High School in El Cajon, California, according to the National Association of School Resource Officers.
Willey's uncle provided a family statement regarding the tragic incident, "This morning, our family was devastated to learn that our handsome Jaelynn was one of the victims in a school shooting at her high school, Great Mills".
Detectives also were gathering information on indications of a "prior relationship between the shooter and the female student", Cameron said.
Officials didn't confirm whether Gaskill hit the attacker, 17-year-old Austin Wyatt Rollins, or whether Rollins shot himself with the Glock semi-automatic handgun he brought to the school.
The sheriff said the school resource officer engaged the shooter and ended the threat, which occurred in a hallway just before classes began.
"He responded exactly as we train our personnel to respond", he said.
"What we know is that the research and data doesn't support the use of school resource officers, whether they are armed or not", said Marc Schindler, executive director of the Justice Policy Institute, a criminal justice reform advocacy group.
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That massacre sparked a new student movement against gun violence, including a national school walkout last week that included some Great Mills students.
Freese said, "I didn't really expect for this to happen".
The incident comes less than a month after local news website TheBaynet.com reported that they had received messages from concerned parents about a threat of a shooting circulating the school. "I feel like if our (school resource officer) would have went in the building, the overall result would've been less bad".
Great Mills High was expected to remain closed Wednesday, and possibly the rest of the week.
Republican Gov. Larry Hogan, meanwhile, accused the Democrat-led legislature of failing to take action on "one of the most aggressive school safety plans in the country". "But we don't have action", said the No. 2 U.S. House Democrat, Steny Hoyer, who represents the area in Congress. A shooter in Parkland, FL, killed 17 people on February 14 at Marjory Stoneman Douglas High School.
Maryland State police and the ATF said they are helping St. Mary's County Sheriff's office in the investigation.
An armed school resource officer had also been on the campus of Stoneman Douglas at the time of that shooting, and was criticized for failing to stop the gunman, who was armed with an AR-15 assault-style rifle.
"The notion of "it can't happen here" is no longer a notion".
Great Mills is located about 66 miles south of Washington, DC, and about 1,500 students attend the high school.