When a gunman attacked a high school in Parkland, Fla., on Wednesday, killing 17 people, the event marked a depressingly familiar milestone.

As of this week, seven of the 10 deadliest mass shootings in modern U.S. history have all happened since 2007. The Parkland massacre is now the eighth-deadliest attack.

The nation’s mass-shooting problem seems to be getting worse. And the latest, most serious shootings all seem to have one new thing in common: the AR-15 semi-automatic assault rifle.

The AR-15 that typically has large magazines, shoots rounds at higher velocities than handguns, and leaves more complex wounds in victims.

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In each one of the older shootings on the 10-deadliest list — including the 2007 attack on Virginia Tech in Blacksburg, Va., that left 32 victims dead — the shooters carried handguns. (The exception is the 1984 San Ysidro, Calif., massacre, where the gunman also used a shotgun and an Uzi semi-automatic carbine.)

But in all of the latest incidents — Newtown, Conn., in 2012; San Bernardino, Calif., in 2015; Orlando, Fla., in 2016; Las Vegas, 2017; Sutherland Springs, Texas, 2017 — the attackers primarily used AR-15 semi-automatic rifles.

There are a couple of theories that might suggest why AR-15s would be associated with deadlier attacks. AR-15 rifles shoot small but high-velocity .223-calibre rounds that often shatter inside victims’ bodies, creating more devastating injuries than the wounds typically left by larger but lower-velocity handgun rounds.

David Hogg, a senior at Marjory Stoneman Douglas High School recorded video of his barricaded classmates during Wednesday's massacre in Parkland, Fla. One day later, he said, "this is completely unacceptable." (The Associated Press)

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Shooters also commonly use the rifles with 30-round magazines, which allow a shooter to fire more rounds uninterrupted, compared to the smaller magazines commonly used in handguns.

Democrats and Republicans are taking different tacks in responding to Wednesday's school massacre. House Minority Leader Nancy Pelosi is pushing legislation, while House Speaker Paul Ryan said, "we need to think less about taking sides." (The Associated Press0

Details were still coming in about Wednesday’s shooting in Florida, but officials say the suspected gunman, Nikolas Cruz, 19, was armed with at least one AR-15 semi-automatic rifle and “countless magazines,” according to Broward County Sheriff Scott Israel.

There’s one thing that does make the Parkland massacre unusual. Of the 10-deadliest mass shootings, Cruz is the only suspected gunman to escape, survive and likely face trial — giving the public a chance to learn why the massacre happened.