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A SWAT-trained school resource officer engaged and fired upon a school shooter at a Maryland high school Tuesday, ending an attack that critically wounded two students. The suspect was killed.

Just before classes were set to begin Tuesday at Great Mills High School, 17-year-old student Austin Wyatt Rollins used a semiautomatic handgun to shoot a 16-year-old girl, who he had a prior relationship with, and a 14-year-old boy in the hallway, according to St. Mary's County Sheriff Tim Cameron.

A school resource officer, Deputy First Class Blaine Gaskill, then faced off with the shooter, the sheriff said. The deputy and the gunman each fired one shot simultaneously. Cameron said the gunman later died from his wounds. He said that the investigation would determine whether the round the officer fired hit the shooter. 

Cameron said it's possible the gunman died of a self-inflicted gunshot wound, but investigators will review surveillance video and determine what happened.

The 14-year-old boy who was wounded and is in stable condition. The 16-year-old girl is in intensive care with life-threatening injuries, Cameron said.  

Officers in St. Mary's have undergone additional training to prepare individual officers to confront shooters, according to Cameron. He said that while he couldn't speak to the shooter's motive or target, "What we do know is the [school resource officer] did his job, engaged, and stopped the potential threat."

The responding SRO Deputy Gaskill is a member of the St. Mary's County SWAT team, CBS News reports.