At least one gunman opened fire following a confrontation with and administrator. The preliminary investigation indicated 27 people were killed, including 12 to 18 children ages 5 to 10.
The incident was among many multiple killings at U.S. schools over the years.
On Valentine's Day 2008, Steven Kazmierczak stormed a lecture hall at Northern Illinois University, killing six. He had been treated for mental illness but apparently had stopped taking his medication.
In 2007, Seung-Hui Cho shot and killed 32 people and wounded 17 others in two attacks at Virginia Tech where he was an English major. Cho had been treated for an anxiety disorder.
In 1999, two seniors at Columbine High School in Columbine, Colo., Eric Harris and Dylan Klebold, shot and killed 12 students and one teacher. The boys both kept journals indicating they had plans to rival the 1995 Oklahoma City bombing.
In 1988, Laurie Dann opened fire in the Hubbard Woods School Elementary in Winnetka, north of Chicago, killing a boy and wounding two girls after a wild day in which she tried to poison two other children and took a family hostage before killing herself. Signs of deteriorating mental health had been noted for a year before the attacks.
In 1966, ex-Marine Charles Joseph Whitman, an engineering student at the University of Texas, opened fire from the school's Tower, killing 13 people and wounding 32 others.
The deadliest school incident in U.S. history was the 1927 Bath Consolidated School bombing in Bath Township, Mich., that left 38 elementary school children dead and 58 people injured. The bombing was committed by a school board treasurer enraged by a property tax increase to fund construction of a new school.