Armed school resource officer stops Md. high school shooter in his tracks. Now the shooter is dead.

An armed school resource officer intervened in a shooting incident Tuesday at a Lexington Park, Maryland, school.

A would-be killer is dead as a result.

What are the details?

Classes had barely begun Tuesday at Great Mills High School when a male student reportedly opened fire in the hallway and struck two classmates: one male and one female.

According to the Washington Post, students and faculty "quickly notified" the armed resource officer on campus.

The officer intervened, and exchange gunfire with the shooter.

Tim Cameron, sheriff of St. Mary's County, told reporters that the armed student died after exchanging shots with the school officer.

The officer was reportedly uninjured.

A male student was hospitalized and in stable condition. A female student — also hospitalized — is in critical condition.

The school was on lockdown during the incident, and students were later evacuated from the school, and shuttled to a nearby high school to be reunited with their parents and guardians.

'We realized our worst nightmare'

"On this day we realized our worst nightmare," Cameron said. "The notion of 'it can't happen here' is no longer a notion."

Cameron said that the shooter's motivation remains unclear.

"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."

Cameron added that there was "no question" that the incident could have been worse if the officer hadn't intervened.

"In my mind, it’s a school shooting in the classical sense, but the investigation will have to determine if there was any connection between the shooter and the victims," Cameron said.

Great Mills, Maryland, is about 70 miles southeast of Washington, D.C.