St. Mary’s County Sheriff Tim Cameron said an armed school resource officer (SRO) ended the shooting at Great Mills High School in Great Mills, MD, on Tuesday morning.

The officer is a trained and armed deputy sheriff.

From NBC Washington:

The gunman entered Great Mills High School in Great Mills at the beginning of the school day and shot a female student in a hallway, Sheriff Tim Cameron told News4. A male student also was hit by a bullet.

Two students are in critical condition, and the shooter was pronounced dead later Tuesday morning.

Information was not available immediately on the relationship between the students, Cameron said. A motive is not yet clear.

The SRO received notification of the shooting and “quickly engaged with the shooter.” He was not injured.

Cameron relayed to the network that his department has prepared for these emergencies:

“You train to respond to this and you hope that you never ever have to,” he said. “This is the realization of your worst nightmare — that, in a school, that our children could be attacked. And so as quickly … as that SRO responded and engaged, there’s grievous injuries to two students.”

He added, “Now begins the second phase of this operation, and that’s the background and the investigation and the attempt for the school to return to normal.”

The sheriff also stressed “that the situation would have been worse if the SRO had not engaged the shooter.”

A month ago, cops investigated shooting threats towards the high school after parents told them of a Snapchat post that warned of a planned shooting. Principal Dr. Jake Heibel told parents and guardians that the administration and the SRO interviewed the student and they concluded “[T]he threat was not substantiated.”

Authorities have not released information about the shooter so we do not know if he/she is from the Snapchat post.

*This is a developing story. More to follow…