Guys and guns: Why men are behind the vast majority of America's gun violence

Several experts said it comes down to America's dangerous combination of toxic masculinity and with gun availability.

This report is a part of "Rethinking Gun Violence," an ABC News series examining the level of gun violence in the U.S. -- and what can be done about it.

Just before 11 p.m. on an April night, a 19-year-old man arrived at his former job site, an Indianapolis FedEx facility, and chatted with security about his status at the company.

The young man then exited his car with two legally purchased rifles and opened fire indiscriminately, at employees inside and outside the building, authorities said.

After killing eight employees -- ranging in age from 19 to 74 -- and injuring at least seven others, the gunman died by suicide, an attack "which he believed would demonstrate his masculinity and capability of fulfilling a final desire to experience killing people," the FBI in Indianapolis said this summer, months after the attack.