Three notable news events that changed the course of history mark important anniversaries this week.
20 years ago: Columbine shooting
The Columbine (Colo.) High School shooting, which happened April 20, 1999 was, at the time, the deadliest high school shooting in American history and prompted a national debate on gun control and school safety. The gunmen, Eric Harris, 18, and Dylan Klebold, 17, killed 13 people and wounded another 20 before turning the guns on themselves.
Investigators later said journals left behind by Harris and Klebold showed they had been planning to bomb their school in an attack similar to Timothy McVeigh's 1995 bombing of the Oklahoma City Federal Building. Instead, they used guns to randomly shoot people.
On April 16, 2007, the shootings at Virginia Tech replaced Columbine as the deadliest school shooting, with 32 people killed by a gunman.
30 years ago: Tiananmen Square protests
On April 15, 1989, Hu Yaoban, a former Communist party leader, died. Hu was trying to move China into a more open political system. Just three days later, thousands of students seeking a more democratic form of government marched in Tiananmen Square in the heart of Beijing.
About a month later, Premier Li Peng imposed martial law to try to control the more than 1 million people who were now rallying in the square. In June, in what would become an iconic image of the protests, a lone man stood alone on the street, blocking a column of Chinese tanks. He was eventually pulled away by onlookers.
Chinese forces stormed the square June 4-5, 1989, arresting 10,000 people and, according to the BBC, killing 10,000 more.
50 years ago: Sirhan Sirhan's conviction
On April 17, 1969, Sirhan Sirhan was convicted of killing U.S. Sen. Robert F. Kennedy, a Democratic candidate for president. On June 5, 1968, Sirhan shot Kennedy three times, once in the head and twice in the back, following a speech the candidate made at a Los Angeles hotel . Sirhan later told authorities he shot Kennedy because of Kennedy's support for Israel during the 1967 Six-Day War.
Sirhan was initially sentenced to death in the gas chamber, but his sentence was eventually commuted to life in prison after California declared the death sentence to be cruel and unusual punishment. Sirhan is now imprisoned in San Diego, and he'll be up again for parole in 2021.