On a "prayer wall" at the church, people came by and wrote in chalk anonymous prayers for the families and students affected by the shooting.
Gaskill contained the situation within one minute of the initial shot being fired, officials said.
"This morning, our family was devastated to learn that our lovely Jaelynn was one of the victims in a school shooting at her high school, Great Mills".
It's not yet clear whether the shooter, Austin Wyatt Rollins, was felled by the officer's bullet or killed himself.
"In the hours to come, in the days to come, through detailed investigation, we will be able to determine if our school resource officer's round struck the shooter", he said. He also shot a 14-year-old boy in the leg in the same hallway.
Police said Tuesday that Willey's injuries are critical. Hogan said no parent should have to worry when they send their kids to school if they will return safely.
"In that split second, a decision has to be made", Cameron said of the standoff.
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".
It commits $125 million for capital improvements like secure doors and windows, metal detectors and security cameras.
While a motive in the shooting can not yet be confirmed, Sheriff Cameron said the initial investigation indicated that there was a prior relationship between the shooter and the 16-year-old girl who was shot.
A school resource officer confronted Rollins, firing at the teen in the school hallway at nearly the same time the student shot his own weapon, said St. Mary's County Sheriff Timothy K. Cameron.
"They're all different", Hoyer said of the school shootings that have occurred this year at a rate of "approximately two a week", but he added, "there is a common element".
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"This is what you hope never happens, but this is what you prepare for", Cameron said.
When a 17-year-old gunman walked into Maryland's Great Mills High School on Tuesday, the swift action of the school's sole resource officer, Blaine Gaskill, was instrumental in bringing the incident to a quick end.
Cameron says all three were hospitalized and the shooter was later pronounced dead.
A school resource officer is credited with potentially saving lives when he confronted the shooter. "During that engagement, he fired a round at the shooter".
The principal at the Maryland high school where there was a shooting Tuesday morning told parents last month that the school had investigated threats of a possible shooting and found they were "not substantiated".
Scott Smith, superintendent of St. Mary's public schools, noted the painful aftermath that will follow the shooting.
Shots rang out just before class began Tuesday morning at Great Mills High School, in Maryland. He was in stable condition in hospital.
Federal Bureau of Investigation agents are on the scene of a shooting at a high school in southern Maryland.
"And on this day, we realize our worst nightmare: that our greatest asset - our children - were attacked in one of our places, a bastion of safety and security, one of our schools". Gaskill is also a member of the local SWAT team and joined the sheriff's office six years ago. The violence hasn't abated since the massacre in Parkland, Florida; A high school student in Birmingham, Alabama, was killed this month when a classmate fired a gun inside a classroom.
Charles County sheriff's spokeswoman Diane Richardson said the sheriff's office forensic unit responded to the scene as well.
In a statement, the uncle, Timothy Cormier, described Willey as an "amazing young lady, whose peaceful presence and love of her fellow students and family" is well-known at the school.
Toni Foreman, who lives across the street, said she was shocked to learn that Rollins might have been involved in the shooting.