As investigators continue to sift through the carnage at Pulse nightclub in Orlando, Florida, familiar details are emerging about the man responsible for the deadliest mass shooting in U.S. history.

Omar Mateen, 29, the U.S. citizen who killed 49 people and injured 53 early Sunday, was driven by radical hatred that he took out on the LGBT community. Authorities were investigating the extent of his connections to Islamic terrorism, but it's clear the killer committed his attack with a powerful assault-style rifle that's available to pretty much anyone who wants one.

This adds to a disturbing trend. According to some counts, there have been nine mass shootings in the past year, including a massacre at a church in Charleston, South Carolina, the anniversary of which will be observed this week. In six of those mass shootings, perpetrators were armed with assault-style rifles. In a seventh -- the November attack on a Planned Parenthood clinic in Colorado Springs, Colorado that killed three and wounded nine -- the shooter was widely reported to have been armed with an AK-47-style rifle, along with other guns that authorities haven't specified.

There's plenty of debate over what constitutes a mass shooting -- as well as what drives those who commit them. But it's increasingly clear that individuals who want to inflict mass casualties are taking advantage of the widespread availability of weapons of war.

These weapons are meant for use on the battlefield.

Renowned firearms engineer Eugene Stoner developed the first AR-15, or ArmaLite Rifle (hence the "AR"), in the late 1950s, using advances in technology and materials to revolutionize battlefield weaponry. Stoner's rifle was marketed to national militaries interested in a lightweight firearm with precision accuracy and high lethality at long range. Colt Manufacturing Co. eventually purchased ArmaLite and convinced the U.S. military to replace the M-14 with its M-16, which employed many of the advances in the AR-15.