The Baltimore Riot of 1968 was primarily caused by the death of Martin Luther King Jr. on April 4, 1968. The riot in Baltimore didn’t start until two days after King’s death.
The riots got so out of hand that Governor Spiro Agnew had to call in the National Guard and Maryland State Police. When these measured fail to halt the riots Agnew had to request Federal troops from President Lyndon Johnson.
Source: Baltimore Police History