Events have been held in the US state of Colorado to mark 20 years since the Columbine High School massacre.

On April 20th 1999, two students went into their school and killed 12 classmates and a teacher.

They also placed two duffel bags containing 20-pound propane bombs set to explode in the school cafeteria.

When they failed to detonate, the two teenagers began their shooting spree.

Eric Harris (18) and Dylan Klebold (17) then took their own lives.

It was one of the deadliest school shooting incidents in US history.

A patrol car is parked in front of Columbine High School in Littleton, Colorado - where two student killed 12 classmates and a teacher in 1999 | Image: Joe Mahoney/AP/Press Association Images

Those killed at Columbine were:

• Matthew Kechter
• Rachel Scott
• Cassie Bernall
• Kyle Velasquez
• Daniel Rohrbough
• Dave Sanders
• John Tomlin
• Corey DePooter
• Kelly Fleming
• Isaiah Shoels
• Lauren Townsend
• Daniel Mauser
• Steven Curnow

Attendees walk through the Columbine Memorial during a vigil at the site for the victims of the massacre at Columbine High School nearly 20 years ago in Littleton, Colorado | Image: David Zalubowski/AP/Press Association Images

Former principal Frank DeAngelis was among those at a faith-based memorial service at a community church on Thursday.

Family members and survivours also attended a vigil at the site for the victims of the shooting on Friday.

A Columbine Memorial was dedicated and opened to the public on September 21st in 2007.

Each of the victim's families were asked to provide a unique and personal reflection in text for the memorial.

These remembrances were engraved in stone and stand as a tribute to the victims.

Tom Tonelli, facing camera, a teacher at Columbine High School, hugs former principal Frank DeAngelis during a faith-based memorial service for the victims nearly 20 years earlier at a community church in Littleton, Colorado | Image: Rick Wilking/AP/Press Association Images

While the Columbine Memorial Foundation - a non profit organisation - was formed in March 2009.

However, it seems the tragedy is never too far from the headlines.

Last week, a woman said to be "infatuated" with the massacre bought a shotgun and ammunition.

Sol Pais (18) was considered "armed and dangerous" and was last seen in the foothills of Jefferson County.

A number of schools in Denver were closed as a result. However she was found dead hours later.

Dave Cullen is a journalist and author of Columbine.

He told Newstalk Breakfast with Susan Keogh the shooting shocked America due to its scale.

But he said gun laws there are actually "getting worse" since.

Main image: Frank DeAngelis, (centre), who was principal of Columbine High School during the massacre nearly 20 years earlier, fights back tears during a faith-based memorial service for the victims of the massacre at a community church, in Littleton, Colorado | Image: Rick Wilking/AP/Press Association Images