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.
Cameron said Rollins fired a Glock 9-mm gun at a 16-year-old girl and 14-year-old boy.
Rollins and the officer fired shots "almost simultaneously", although it is unclear how he died.
Authorities initially said Rollins shot the male victim after shooting the female victim but later said they needed to investigate further before determining if he shot both.
The officer ordered the student to drop the gun again, before two shots sounded, marking the end of the latest school shooting for a nation tired of them. Heibel said the school increased its security nevertheless after social media posts about a possible school shooting "circulated quite extensively". Deputy Gaskill is credited with containing Tuesday's high school shooting within a minute of Rollins' initial gunshot being fired.
"We recently had a protest about school violence last week, and now this has happened", said Kameron Norwood, 16, as he and other students waited for relatives to pick them up from a nearby high school.
"There was a relationship prior to this event", the sheriff said.
The school has about 1,600 students and is near the Patuxent River Naval Air Station, about 65 miles (104 kilometers) southeast of Washington.
Sen. Benjamin Cardin, D-Md., said, "having a trained, professional school resource officer made a difference. armed school resource officers are available in our high schools and that's important". The female student was taken to PG Shock Trauma, where she is in ICU with life-threatening critical injuries.
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While the 14 year-old victim's condition has been described as "good" by local news and authorities, Willey is now in critical condition with life-threatening injuries at UM Prince George's Hospital Center and the shooter has been declared deceased.
In July 2016, Gaskill was responding to a complaint at a Great Mills home when a man - identified as Pekka Robert Heinonen, 59 - pointed a gun at him, according to Maryland newspaper The Enterprise.
Witnesses to the shooting were questioned at a secured area in the high school, the sheriff said.
Several students from Stoneman Douglas tweeted about the Great Mills school shooting on Tuesday, offering their thoughts and calling for an end to gun violence in schools.
The suspected gunman, 17-year-old student Austin Wyatt Rollins, was pronounced dead hours later at a local hospital.
"You never think it'll be your school and then it is", Mollie Davis, who identified herself as a student at the school, wrote on Twitter.
During a press conference, the St. Mary's County Sheriff's Office said that Rollins walked into Great Mills High School with a gun.
Gov. Larry Hogan (R), said that afternoon of Gaskill, "This is a tough guy who closed in very quickly" on the situation.
"It is hard for us not to see her shining, smiling face right now, and to see her light up the room with her presence", Willey's family said in a statement. Julie Yingling says there have been no fatalities in the Tuesday morning shooting at Great Mills High School. The shooting happened in the entrance of the school in hallway F.