Aftermath info of the columbine shooting?

After the shooting, the police tallied 15 dead (including the shooters) and 21 wounded. Most people thought it couldn't have possibly been worse. But...

It was found out days after the shooting during the investigation that Dylan and Eric never intended to storm the school; they had planted propane bombs in the cafeteria that were set to go off shortly after lunch had started. The ensuing fires would've inevitably forced students out of the side exits, at which point Dylan and Eric would've gunned them down. The death toll would've been much higher had the propane bombs gone off. Thankfully, they didn't.*

The shooting - as terrible as it was - lead to increased security in schools and new protocols added in every North American school, lest that sort of thing happen again (For instance, schools now have a "lockdown" policy - in the event that a shooter or other threatening party enters the building, all the classrooms lock their doors and do not open them until an official announcement comes over the loudspeakers).

* - One of the propane bombs placed in the cafeteria actually did go off, but only after one of the shooters shot it.