As people react in anger and sadness to Friday's shooting spree at an elementary school in Newtown, Conn., police are still looking for answers about what prompted the incident.
The shootings – which killed 27 people, including 20 children – are just the latest in a series of violent outbursts at U.S. schools and universities over the past few decades.
The Connecticut shooting is at least the 18th mass shooting at a school or university in the United States during the past 46 years. Some have occurred in Ohio and Kentucky.
Here is an overview of the incidents.
** April 2, 2012 – One L. Goh, 43, allegedly fired a .45-caliber semi-automatic handgun at Oikos University in Fullerton, Calif. He is accused of killing seven people and injuring three others, before fleeing the scene. Goh drove to a supermarket, where he surrendered to police.
Goh has pleaded not guilty to the charges and is currently awaiting trial.
** Feb. 27, 2012 -- T.J. Lane, 17, used a .22-caliber semiautomatic Ruger handgun taken from his uncle's home to kill three students and injure two others in a cafeteria at Chardon High School in Chardon, Ohio.
Lane, who is being tried as an adult, faces three counts of aggravated murder, two counts of attempted aggravated murder and one count of felonious assault. He has pleaded not guilty by reason of insanity. Lane's trial is slated to begin Jan. 14.
** Feb. 14, 2008 -- Steven Kazmierczak, 27, entered a lecture hall at Northern Illinois University in DeKalb, Ill., killing five people and injuring 21 others, before committing suicide.
** April 16, 2007 -- Seung-Hui Cho, 23, kills 32 people before committing suicide on the Virginia Tech campus in Blacksburg, Va.
** Oct. 2, 2006 – Charles Carl Roberts IV, 32, took a Springfield XD .9mm handgun and entered an Amish one-room schoolhouse in Bart Township, Pa. Roberts killed five girls and injured five others, before committing suicide.
** March 21, 2005 – Jeffrey Weise, 16, uses a .22-caliber pistol to kill his grandfather and his grandfather's girlfriend. Weise then took his grandfather's .40-caliber Glock 23 pistol and a Remington 870 12-gauge shotgun and drove to Red Lake Senior High School in Red Lake, Minn.
There, Weise killed 5 students, a teacher and a security guard, and injured 5 students. After a shoot-out with police, he committed suicide.
** April 20, 1999 -- Eric Harris, 18, and Dylan Klebold, 17, opened fire at Columbine High School in Littleton, Colo., killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library.
** May 21, 1998 – A day after killing his parents with a .22 Ruger rifle, Kip Kinkel, 16, took a .9mm Glock 19 pistol, a Ruger .22-caliber semi-automatic rifle and a .22-caliber Ruger MK II pistol to Thurston High School in Springfield, Ore., where he killed two more people and injured 25 others.
Kinkel later pleaded guilty to murder and attempted murder just before a trial was to begin. He was sentenced to 111 years in prison.
** March 24, 1998 – Mitchell Johnson, 13, and Andrew Gold, 11, drove to Westside Middle School near Jonesboro, Ark., where they killed four students and a teacher, and wounded nine other students and a teacher.
The boys were sentenced to confinement until they reached age 21. Johnson was released in 2005, and Golden was released in 2007. As an adult, Johnson later was convicted on various state and federal charges involving weapons and drugs, and is serving 22 years in prison.
** Dec. 1, 1997 -- Michael Carneal, 14, fires a .22-caliber pistol at a youth prayer group meeting before the school days begins at Heath High School in Paducah, Ky. Three people were killed, and five injured. The victims ranged in age from 14 to 17.
Carneal was sentenced to life in prison, with the possibility of parole in 25 years. His attorneys are asking the U.S. 6th Circuit Court of Appeals to withdraw his verdict due to mental illness at the time.
** May 25, 1994 – Clay Shrout, 17, took a .380-caliber pistol and killed his mother, father, and his two sisters in their Florence, Ky., home. Shrout then drove his prom date's house and kidnapped her. Shrout took her to Ryle High School, where he held 22 students and a teacher hostage. The ordeal ended after 17 minutes when Shrout surrendered to a police officer.
Shrout pleaded guilty but mentally ill and was sentenced to live in prison.
** May 1, 1992 -- Eric Houston, 20, went to Lindhurst High School in Olivehurst, Calif. There, he used a 12-gauge shotgun and a .22-caliber rifle to kill three students and one teacher, and injure 10 other people.
Houston was found guilty on multiple charges and sentenced to death. He is currently awaiting execution.
** Nov. 1, 1991 – Gang Lu, 28, went to the University of Iowa in Iowa City, Iowa, and opened fire with a .38-caliber revolver. Lu killed four faculty members and one student, and injured another student. Afterward, he committed suicide.
** Jan. 17, 1989
– Patrick Edward Purdy, 24, opened fire with a Type 56 Assault Rifle at Cleveland Elementary School in Stockton, Calif. He killed five students, and injured 29 other students and one teacher. Purdy then used a pistol to commit suicide.
** Jan. 29, 1979 – Brenda Spencer, 16, took a .22-caliber rifle that her father had given her as a Christmas present and went to a window in her San Diego, Calif., home. She then began firing at Grover Cleveland Elementary School across the street. She killed a principal and a teacher, and injured eight students and a police officer.
Spencer pleaded guilty to murder, and is serving a prison sentence of 25 years to life.
** July 12, 1976 -- Edward Charles Allaway, 37, a custodian at California State University, Fullerton, killed seven co-workers and wounded two others.
Allaway was found not guilty by reason of insanity and committed to a mental institution. In 2001, his psychiatrists said his paranoid schizophrenia was in remission, but a judge rejected his request to be released.
** Aug. 1, 1966 – Charles Whitman, 25, climbed a clock tower at the University of Texas in Austin, carrying various weapons including three rifles and a shotgun. He killed 16 people and injured 32 others, before being shot to death by a police officer.
(Sources: WCPO Archives, Associated Press, Encyclopedia of Murder)