ATLANTA — The call was for an active shooting at Marshall County High School in rural Kentucky, and tears welled up in Riley Johnson’s eyes as he prepared the medical evacuation helicopter that he flies.

He blinked them away and flew to the campus, where he had run cross-country as a member of Marshall’s class of 2009. He set the helicopter down in an area that he knew well. And then, as colleagues rushed toward a patient, he surveyed the chaotic scene of panicked teenagers and hurried emergency officials.

“I just had a pretty good breakdown and just started bawling my eyes out in the cockpit, not believing what I was seeing,” Mr. Johnson, 27, recalled in an interview on Wednesday. “I can imagine myself being in that school, exactly where this happened.”

A teenage gunman’s rampage in Benton, Ky., a speck of a city near the Illinois border and the latest American town to confront a mass shooting, left two students dead and 18 other people wounded on Tuesday.