"She fired on him twice and drew the attention toward her. He immediately spun around and charged her," Medley said. "She fired a couple more rounds and fell back, continuing to fire."
Despite getting struck three times by Hasan's fusillade, Munley stayed upright and kept firing at the charging gunman.
"She struck him a couple times in the upper torso, and he went down," Medley said.
"When she rounded that corner she made a split-second decision to put her life at risk," he said.
Lt. Gen. Robert Cone said Munley's aggressive tactics averted even more carnage.
"She had been trained in active response," Cone said. "They had rehearsed scenarios like this. Oftentimes, the idea is you would encircle the building and wait until you have more backup. What the belief is, if you act aggressively, to take the shooter out, you'll have less fatalities."
Munley acted aggressively, not waiting for backup. She went after the gunman and quickly found him. As Cone put it, Munley decided "to seek him out, to confront hm."
Munley's grandmother, Monirie Metz, told ABC News that the former South Carolina surfer girl would probably object to being called a hero.
"Kim doesn't want be called a hero. She's worried about everyone else right now and is very concerned about her colleagues with whom she is very close," Metz said.
Her husband, Matthew Munley, is a soldier at Fort Bragg, N.C., and was flown to Fort Hood. She also has two daughters, ages 15 and 2, from a previous marriage.
ABC News' Mark Mooney contributed to this report.