- Multiple people were shot at out of which two are critically injured
- Shooting took place at Great Mills High School in St Mary's County
- School officials confirmed campus was on lockdown and incident contained
The shooting, which came amid a renewed national debate over gun violence following last month's Florida high school massacre, occurred just before 8 a.m. (1200 GMT) at Great Mills High School in St. Mary's County, about 70 miles (110 km) south of Washington.
The 16-year-old girl and a 14-year-old boy who were wounded were taken to hospitals, county Sheriff Timothy Cameron said. The girl was in intensive care with life-threatening critical injuries, while the boy was in good condition, the sheriff said.
The gunman was identified as Austin Wyatt Rollins, and Cameron said there was "an indication" of a prior relationship between him and the female student, though he added that was still under investigation.
The latest in a long string of deadly shootings at US schools and colleges took place a little more than a month after 17 students and educators were shot dead at Marjory Stoneman Douglas High School in Parkland, Florida.
That massacre sparked a new student movement against gun violence, including a national school walkout last week that included some Great Mills students. It occurred just days before a planned Saturday march in Washington calling for new restrictions on guns.
"We recently had a protest about school violence last week, and now this has happened," said Kameron Norwood, a 16-year-old sophomore, as he and other students who had been transported to another nearby high school waited to be picked up by relatives.
Sheriff Cameron said Rollins pulled out a Glock semiautomatic handgun around 7:55 a.m. (1155 GMT) in a hallway and shot the students.
The attack, which lasted less than a minute, ended after the school resource officer, Deputy 1st Class Sheriff Blaine Gaskill, ran inside the building and confronted Rollins, with both firing a single shot almost simultaneously.
The officer was not harmed, Cameron said. Rollins was confirmed dead at 10:41 a.m. (1441 GMT) after being taken to a hospital.
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 officer, who resigned, said he had not been sure where the gunfire was coming from.
US President Donald Trump and the National Rifle Association have proposed arming teachers trained in firearms use to combat the threat of school shootings, while gun safety advocates have demanded a ban on semiautomatic rifles, among other laws.
More than 29,000 public schools in the United States, or roughly 30 percent, reported having at least one full- or part-time school resource officer in 2013, according to a 2015 survey by the National Center for Education Statistics.
Rollins was a fan of the Dallas Cowboys football team and NASCAR racing, according to his Facebook page, where he also "liked" the sort of tastes typical of an American teenager: McDonald's, Wrangler jeans, the "Transformers" movie and Sour Patch Kids candy. He appeared several times on the school's honour and merit rolls for good grades, according to lists published in a local newspaper.
Parkland students and Great Mills students exchanged supportive messages on Twitter following Tuesday's shooting.
"We are here for you, students of Great Mills, together we can stop this from ever happening again," tweeted Emma Gonzalez, a senior at Stoneman Douglas High School who survived last month's rampage.
"You never think it'll be your school and then it is," posted Mollie Davis, who identified herself as a student at the school and tweeted during the lockdown. "Great Mills is a wonderful school and somewhere I am proud to go. Why us?"
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