Run, hide or fight.
That's what authorities say is the way to survive a mass shooting or active-shooter situation, said John Bruner, a 20-year law-enforcement veteran who was the former chief county detective at the Greene County (Pa.) District Attorney's Office.
When you've tried or can't do those things, play dead, said Bruner. That animal trick can work.
Ask Zachary Emenegger, of Las Vegas. In June 1999, Emenegger, then 21, worked the overnight shift at an Albertson's supermarket when a man named Zane Floyd entered the store and began shooting people.
After getting shot twice at close range, Emenegger pretended he was dead. He survived, although he sustained critical injuries. Floyd was eventually arrested and convicted of murdering four supermarket employees. He is on death row.
Watch the full story on "20/20: In an Instant" Friday at 10 ET.