Here is some background information about the deaths of 13 people at Columbine High School in Littleton, Colorado, on April 20, 1999.
Facts: Twelve students and one teacher were killed by students Dylan Klebold, 17, and Eric Harris, 18.
The pair made home videos prior to the attack making references to what they were going to do and apologizing to their parents for it.
Harris and Klebold killed themselves with gunshot wounds to the head in the school's library at approximately 12:08 p.m. on the day of the shootings.
SWAT teams entered the school 47 minutes after the shootings started. Five hours passed before law enforcement declared the school under control.
Cassie Bernall, 17
Steven Curnow, 14
Corey DePooter, 17
Kelly Fleming, 16
Matthew Kechter, 16
Daniel Mauser, 15
Daniel Rohrbough, 15
William "Dave" Sanders, 47
Rachel Scott, 17
Isaiah Shoels, 18
John Tomlin, 16
Lauren Townsend, 18
Kyle Velasquez, 16
Timeline: January 1998 - Dylan Klebold and Eric Harris are arrested after stealing items from a van. After pleading guilty, they are sent to a juvenile diversion program.
March 1998 - Randy and Judy Brown, parents of student Brooks Brown, file a report with the sheriff's office stating that Harris had threatened to kill Brooks and had written on the internet that he would like to kill people.
April 20, 1999 - At approximately 11:19 a.m., two students, Dylan Klebold, 17, and Eric Harris, 18, carrying guns and bombs, open fire inside Columbine High School, killing 13 and wounding 23 others before killing themselves.
November 12, 1999 - Mark Manes is sentenced to six years in prison for selling a gun used in the murders to minors Harris and Klebold.
April 2001 - Close to three dozen families of Columbine victims settle suits with the parents of the suspects and gun suppliers. The settlement totals close to $2.5 million. The Harris' and Klebold's homeowners insurance will pay a large part and the rest will come from insurance company payments on behalf of the gun suppliers. The family of victim Isaiah Shoels does not accept the settlement.
August 20, 2002 - The Jefferson County Sheriff's Office settles with the daughter of teacher Dave Sanders for $1.5 million.
June 2003 - Judge Robert Blackburn orders the family of Isaiah Shoels to accept a $366,000 settlement in the lawsuit against the gunmen's families.
August 12, 2003 - The families of victims Daniel Rohrbough, Kelly Fleming, Matt Kechter, Lauren Townsend, and Kyle Velasquez settle a wrongful death lawsuit against parents Susan and Thomas Klebold and Wayne and Katherine Harris, in which the victims' families claim that the suspects' parents should have known what their sons were up to before the shootings. The terms of the settlement have not been released.
October 22, 2003 - Home video of the two suspects is released by authorities. In the video, made six weeks before the murders, the suspects are seen in a forested area shooting at bowling pins.
February 26, 2004 - Colorado Attorney General Ken Salazar releases an investigative report about the attack. Authorities also release thousands of pages of documents and physical evidence.
September 21, 2007 - The Columbine Memorial, adjacent to Columbine High School, is dedicated and opened to the public.
February 12, 2016 - In the first television interview since her son Dylan killed 13 people at Columbine High School, Susan Klebold speaks to Diane Sawyer. Klebold states that "If I had recognized that Dylan was experiencing some real mental distress, he would not have been there," she says. "He would've gotten help. I don't ever, for a moment, mean to imply that I'm not conscious of the fact that he was a killer, because I am."