April 15 - In 1912, the British luxury liner Titanic sank in the North Atlantic off Newfoundland, less than three hours after striking an iceberg. About 1,500 people died.
April 16 - In 1818, the United States Senate ratified the Rush-Bagot Treaty, establishing the border with Canada.
April 17 - In 1961, about 1,500 CIA-trained Cuban exiles launched the disastrous Bay of Pigs invasion of Cuba in a failed attempt to overthrow the government of Fidel Castro.
April 18 - In 1906, a devastating earthquake struck San Francisco, followed by raging fires. About 700 people died.
April 19 - In 1995, a truck bomb exploded outside the Alfred P. Murrah Federal Building in Oklahoma City, killing 168 people, and injuring 500. Timothy McVeigh was convicted of the bombing and sentenced to death.
April 20 - In 1999, two young men stormed into a suburban high school in Littleton, Colorado at lunch time with guns and explosives, killing 13 and wounding dozens more in what was at the time the nation's deadliest school massacre.