Published: Apr 20, 2010 11:18 AM EDT

Today marks the 11-year anniversary of one of the deadliest school shootings in history.  In 19-99, two armed students killed 13 people before taking their own lives .
The Columbine High school massacre occurred on on this day, April 20th, back in 1999 around lunchtime. Two students, Eric Harris and Dylan Klebold, shot and killed 12 classmates and one teacher before ending the ordeal by taking their own lives.
It is considered the 4th deadliest school massacre in the United States' history.

Afterward, the shooting started a big debate about gun control laws, bullying, even violent movies and video games.  Schools also began instituting new security measures such as see-through backpacks, metal detectors, and security guards.  Police departments even trained for Columbine-like situations after criticism over response-time.
The father of one shooting victim says, this 11th anniversary will be a quiet one for him as he and his family visit the cemetery where his son is buried.
Tomorrow, actor Tom Arnold will be among those in Denver to honor the Columbine victims, and attend the International School Safety Convention.  The goal of the 3-day conference is to promote school safety in America.