GREAT MILLS, Md. — A teenager with a handgun shot and critically wounded a girl inside a Maryland school on Tuesday and the shooter was killed when a school resource officer confronted him moments after the gunfire erupted.

It wasn’t immediately clear whether the shooter took his own life or was killed by the officer’s bullet, St. Mary’s County Sheriff Tim Cameron said, but the officer was credited with preventing any more loss of life.

Authorities didn’t release a motive, but said they believe the girl and the shooter — 17-year-old Austin Rollins — previously had a relationship.

A 14-year-old boy also suffered a gunshot wound, but it wasn’t clear who shot him. He was in good condition.

The officer, who doubles as a SWAT team member, was unharmed.

The shooting at Great Mills High School rocked a nation still reeling from the Feb. 14 massacre of 17 people at a Florida high school by a teenage boy with an assault weapon. Students across the country have planned an anti-gun violence march this weekend at the nation’s capital.

Politicians responded swiftly to the Maryland shooting, acknowledging that it increased the pressure for action.

“We sympathize. We empathize. We have moments of silence. But we don’t have action,” said the No. 2 U.S. House Democrat, Steny Hoyer, who represents the area in Congress. “Wringing our hands is not enough.”

In this case, it appeared the shooter illegally possessed the gun. In Maryland, a person must be 21 to possess a handgun, unless carrying one is required for employment. It’s not clear how Rollins obtained the weapon.

Attempts to reach his family were unsuccessful.

One of the shooter’s friends, 14-year-old Jordan Hutchinson, and his mother dropped off a condolence card at the Rollins home.

Jordan recalled meeting Austin five years ago during a snow storm, and playing together building snow forts.

“Austin was a nice kid. We did sleepovers all the time,” he said.

The sheriff praised the school resource officer, Deputy First Class Blaine Gaskill, a six-year veteran in his first year at the high school, for containing the situation in less than a minute.

“He had to cover significant ground,” Cameron said. “The premise is simple: You go to the sound of gunfire.”

Students endured a lengthy lockdown, cowering inside classrooms and a locker room while officers worked to make sure there were no more threats on campus.