A gunman fired on a parade in honour of Martin Luther King in Baton Rouge, Louisiana, yesterday, killing one man and seriously injuring three children.
While details of the incident remained sketchy, police said last night that they were searching for at least one assailant believed to be an African American man.
The shooting cast a pall over the annual Martin Luther King holiday which was celebrated across the United States yesterday.
Schools, banks and many businesses closed and for the first time the New York stock markets also stayed shuttered for the day.
"There was fighting and someone pulled out a gun and started firing," Charles Armstrong, a police officer, said. The gunman then ran off.
A police spokesman, Don Kelly, said the gunshots sparked "mass pandemonium" in the crowd watching the parade, which began at a city high school and headed for the city centre.
Police have ruled out a racial motive for the shooting as all four victims were said to be black. A 25-year-old man was killed outright, police said. A six-year-old girl was in a critical condition with a gunshot wound in the back.
Listed in serious condition was an 11-year-old girl with gunshot wounds to the legs and hand. A nine-year-old boy suffered a leg wound.
King, who won a Nobel Peace Prize for his civil-rights efforts, was shot on 4 April 1968 while standing on a balcony of the Lorraine Motel in Memphis, Tennessee.
At the weekend, the city of Memphis launched a three-month schedule of events to mark his life.
President Bill Clinton yesterday helped paint walls in a Washington DC High School to mark the holiday and dispatched many of his Cabinet members to church services and commemorative parades around the country. He also pledged additional government funding for civil-rights enforcement.
The announcements in Memphis on Sunday were marred by a demonstration by 50 members of the Ku Klux Klan who were protesting that Martin Luther King Day was made an official federal holiday by Congress in 1986.
After a slow start, the holiday has increasingly been respected across the country.
Commenting on yesterday's shooting, Eddie Crawford, from the Baton Rouge chapter of the NAACP, the national black- rights association, said: "There has never been an incident like this."
Noting that the parade passed by poor housing in the city, he said: "Any time you have a lot of young people, you're going to have a problem with guns."