Twelve students and a teacher were killed in the shootings on April 20, 1999. The teenage gunmen committed suicide.
Lawmakers in the Colorado House held a moment of silence Wednesday morning and some spoke about how they were affected by the tragedy.
No events were planned at the school but a memorial to the victims in Clement Park near the school is open to the public.
A documentary about the different ways that families who lost loved ones in the shootings premiered in the Denver area this week.
The film doesn't include pictures of Eric Harris and Dylan Klebold, reflecting frustration expressed by families at the attention focused on the gunmen.
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