Colorado observes 20th anniversary of Columbine school massacre

Several mass school shootings since have led to calls to change gun laws in the US.

    Saturday marks the 20th anniversary of the Columbine High School shooting in the United States.

    Twelve students and a teacher were killed when two teenagers carried out a planned attack at the school in Colorado.

    Several mass school shootings since have led to calls to change gun laws in the US.

    Al Jazeera's Alan Fisher reports.


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