Florida shooting: What's driving the increase in deadly school shootings?

Wednesday's attack in South Florida, while shocking, is the type of tragedy we have seen too many times before. 

On Jan. 23, a young gunman opened fire inside a crowded school atrium in Kentucky, killing two students and wounding 14 others. 

"Everybody started running and stuff. I saw people getting shoved down," a witness told CBS News. "There was a lot of blood everywhere. It was horrible." 

On Dec. 7, 2017, at a high school in Aztec, New Mexico, another young gunman shot and killed two of his classmates. 

"Tragically, the horrors that visited many other communities have come to roost here in Aztec," said Michael Paddilla. 

On Nov. 14, 2017, a gunman in Tehama County, California, fired 30 rounds into an elementary school as children hid beneath their desks. None of the students died, but after the gunman was killed by police, they discovered he had earlier murdered multiple people, including his wife. 

"We located her dead body concealed under the floor of the residence," police told CBS News. "We believe that's probably what started this whole event." 

What starts these events is always the subject of detective work, community second-guessing and familial agonizing. Perhaps the biggest question: Why is this happening more frequently? 

This year, there have already been 18 school shootings. There were seven by this time last year. Since 2013, the number of school shootings has steadily risen, with the sole exception being 2016, when there were only 48 -- as if that was in any way acceptable or normal. 

As a sign of the times, more schools now prepare their students for gun violence. Two-thirds of school districts report conducting active shooter drills even though only 19 states require them. 

"This is something that is so prevalent it is actually rehearsed. It is part of the curriculum across the U.S. making sure that we understand how to properly and expeditiously and safely contain this type of situation, and minimize the injuries," said CBS News law enforcement analyst Paul Viollis. 

Researchers say school shootings are happening at a rate of one a week. Some are accidental or suicide, where the only victim was the shooter himself.