The bombs that went off at the finish line of the Boston Marathon Monday marked the fifth violent incident to occur during mid-April on U.S. soil in the past 20 years.
Two people were dead and dozens injured after the blasts went off just before 3 p.m. near the annual race's finish line on Boylston Street, according to police. Authorities believed the blasts were caused by small portable devices, sources told ABC News.
The federal raid on a religious compound near Waco, Texas, 20 years ago this week, the bombing of a federal building in Oklahoma City, 18 years ago this week, the deadly shooting at Columbine High School 14 years ago this week, and the slaying of 32 people at Virginia Tech University six years ago today were four other April tragedies.
The Waco Siege - April 19, 1993
On Feb. 28, 1993, the U.S. Bureau of Alcohol, Tobacco and Firearms (ATF) attempted to serve a search a search warrant at the Mount Carmel Center ranch in Elk, Texas - just northeast of Waco, Texas - owned by the Branch Davidians, a religious group. The attempt led to an eruption of gunfire, leaving four ATF agents and six Branch Davidians dead.
As days stretched into weeks during a tense standoff, the government gradually increased the pressure on the religious group's leader, David Koresh.
After a standoff that lasted 50 days, on April 19, 1993, the FBI led an assault on the compound. U.S. Attorney General Janet Reno approved a plan to fire CS gas - a form of tear gas - into the compound to force the Branch Davidians out. But the Branch Davidians refused to emerge.
Around noon, the first sightings of flames within the ranch were reported. Fanned by high winds, the building was soon engulfed in flames. Within minutes, a number of explosions went off in the building.
At least 74 people - including 25 children - perished in the siege.
Oklahoma City bombing - April 19, 1995
On April 19, 1995, an explosion at the Alfred P. Murrah Federal Building in downtown Oklahoma City killed 168 people - including 19 children under the age of 6 - and injured over 800.
Less than two hours after the explosion rattled the city, Timothy McVeigh, a 26-year-old Gulf War veteran, was arrested. The VIN number from a Ryder box truck rented by McVeigh linked him to the attack.
McVeigh was convicted of conspiracy, using a weapon of mass destruction and murdering eight federal law enforcement officers, and sentenced to death. Convicted co-conspirator Terry Nichols, 46, is serving a sentence of life in prison.
McVeigh was executed by lethal injection on June 11, 2001. The bombing remains the most deadly domestic terrorism attack in U.S. history.
Columbine High School Shooting - April 20, 1999
On April 20, 1999, two teenagers walked into their high school in Littleton, Colo., and killed 12 of their classmates and a teacher before taking their own lives.
The fatal rampage conducted by Eric Harris, 18, and Dylan Klebold, 17, injured 21 more and struck fear into the hearts of parents with school-age children nationwide. It also sparked renewed fears about the exposure of children to violence in video games, television and films and rekindled the debate over gun control laws.
Virginia Tech University Shooting – April 16, 2007
On April 16, 2007, Virginia Tech senior Seung-Hui Cho unleashed a rampage on the Virginia Tech campus, in Blacksburg, Va., shooting and killing 32 students, and wounding 17 more people, making it then the deadliest mass shooting in U.S. history.
More than a year before the massacre, in December 2005, a district court in Montgomery County, Va., deemed Cho "mentally ill" and "an imminent danger to self and others."
The aftermath of the Virginia Tech shooting prompted Congress and President George W. Bush to sign the first major change to U.S. gun laws in more than 10 years -- it expanded the federal background check database -- and overhauled the way many campuses handle crime and security alerts.
PHOTOS: Boston Marathon Explosions