Everytown for Gun Safety, a gun control advocacy group, had recorded 17 school shootings on their website prior to this afternoon's shooting in Parkland, Florida.
Wednesday's shooting marks the first of the year in Florida. There were three shootings at different schools in Texas, two in different California schools and two in different Michigan schools, according to Everytown's data. There are 10 other states that had at least one shooting.
In eight of the 17 school shootings recorded by Everytown, a gun was fired but no one was injured.
Two of the shootings were classified as being attempted or completed suicides with no intent to injure another person.
Schools have been some of the deadliest sites for shootings in the past.
- Sources: At least 17 killed in Florida high school shooting (LIVE UPDATES)
- What to do if you're in an 'active shooter' situation
- How to talk to your child about a traumatic event like a mass shooting
The third deadliest mass shooting in modern U.S. history was at Virginia Tech University in 2007, when 32 people were killed, and the Sandy Hook Elementary School shooting, which is tied for the fourth-highest casualty shooting, led to 26 deaths.
Broward County Public Schools superintendent Robert Runcie said that there were "numerous" fatalities in today's shooting at Marjory Stoneman Douglas High School.
According to Brad Garrett, a former FBI agent and current ABC News consultant, most school shootings last for about five minutes or less.
It takes much longer to clear the scene, however, since responding law enforcement officers need to methodically go through, room by room, to both secure and de-arm the shooter and to help students and faculty at the school, Garrett explained.
Former New York Police Department commissioner Ray Kelly said that bullying could be a possible factor in today's shooting, though the motive has not been confirmed.
"We've seen it in so many cases," said Kelly, who is now an ABC News consultant.
"We don't know for sure but I'm pretty sure there's an element of that here," he said, noting that the bullying could be "real or perceived."
© 2018 Cox Media Group.