At least 15 people, including one teacher, were killed by two teenage gunmen armed with bombs and automatic weapons. A further 23 were injured. The killers, named as pupils Eric Harris and Dylan Klebold, later turned their guns upon themselves.
The Littleton tragedy "will make it almost imperative that the legislatures act," said Myra Reichart, an official of the teachers' union in Edinboro, Pennsylvania, where a teacher was killed by a student in a shooting at a school dance exactly one year ago tomorrow.
Several states began considering gun restrictions last spring following other school shootings.
In Arkansas, where two boys last year killed five people and injured 10 others at their school, the state senate defeated a proposal to make adults liable when they fail to lock up their guns and the weapons are used by children to kill or injure others.
A similar proposal, in Oregon, where two students were killed and 24 injured in a high school shooting rampage last May is being opposed by gun owners. In Colorado, the site of the Littleton shootings, plans to make it easier to carry a concealed weapon have been dropped in response to the massacre.