A memorial ceremony was held Saturday in the American state of Colorado to commemorate the 20th anniversary of the Columbine High School massacre, one of the deadliest school shootings in U.S. history. /Getty Images

People were seen laying flowers, cards as well as seed packets of columbines, the Colorado state flowers at the memorial site. /Getty Images

Scott Christy and Frank DeAngelis, the current and former principals of Columbine High School, gave speeches during the ceremony, reflecting on the traumatic event, in which 13 people were killed. /Getty Images

On April 20, 1999, two heavily-armed 12th grade students – Eric Harris and Dylan Klebold –murdered 10 students and one teacher before both committed suicide, leaving the U.S. in shock and forever transforming the debate on gun rights and school violence across the nation. /Getty Images