April 20 (UPI) -- On this date in history:
In 1871, the U.S. Congress passed the Third Force Act, popularly known as the Ku Klux Klan Act, authorizing President Ulysses S. Grant to declare martial law, impose heavy penalties against terrorist organizations and use military force to suppress the Klan.
In 1902, Marie Curie and Pierre Curie isolated radioactive radium salts from the mineral pitchblende in their laboratory in Paris.
In 1961, Radio Havana announced that seven members of the group which attempted an overthrow of Fidel Castro's communist government in Cuba had been executed. It raised the number of executions over the previous three days to 24.
In 1976, the U.S. Supreme Court ruled that federal courts could order low-cost housing for minorities in a city's white suburbs to ease racial segregation.
In 1983, President Ronald Reagan signed a $165 billion Social Security rescue plan to keep the retirement system solvent.
In 1991, the United States announced plans to open an office in Hanoi to investigate unresolved cases of 2,278 U.S. military personnel listed as MIAs and POWs.
In 1992, Madonna signed a multimillion-dollar deal with Time Warner to form an entertainment company that would make her the world's highest paid female pop star.
In 1999, two teenage boys killed 12 students and a teacher at Columbine High School in Littleton, Colo., before turning their guns on themselves.
In 2008, Danica Patrick won the Indy Japan 300 auto race, becoming the first woman to win an IndyCar event.
In 2010, an explosion and fire on the Deepwater Horizon BP oil rig off the Louisiana coast in the Gulf of Mexico killed 11 workers and caused a massive oil spill. It became the largest U.S. marine oil spill in history, stretching over almost three months and releasing about 4.9 million barrels of crude.
In 2011, Michel Martelly, an entertainer who performed under the name "Sweet Micky," was elected president of Haiti in a runoff with former first lady Mirlande Manigat.
In 2012, a Pakistani Bhoja Air jetliner on a flight from Karachi crashed 5 miles from Islamabad, killing all 127 people aboard.
In 2013, an earthquake in China's Sichuan province killed nearly 200 people and injured thousands.
In 2018, British commonwealth leaders decided Prince Charles will succeed his mother, Queen Elizabeth II, as head of the commonwealth, a role that is not necessarily hereditary and must have unanimous support.