A letter from President Lyndon Johnson to the widow of Martin Luther King was sold for $60,000 at auction on Thursday.

Dated April 5, 1968, the typed letter from Johnson to Coretta Scott King was written the day after King was murdered in Memphis, Tennessee, by white supremacist James Earl Ray, triggering riots in cities across the United States.

Johnson wrote: "We will overcome this calamity and continue the work of justice and love that is Martin Luther King's legacy and trust to us."

Ray was arrested two months later in London and extradited to the US. He pleaded guilty to the crime and was sentenced to 99 years in the Tennessee State Penitentiary, where he died in 1998 at the age of 70.

Quinn's Auction Galleries in Falls Church, Virginia, set a minimum price at $60,000, the sum an online bidder ultimately paid for it, though the item had been expected to fetch at least twice that amount.

Auctioneer Matthew Quinn said the letter had special resonance given the 50th anniversary this month of the "Bloody Sunday" protest march at Selma, Alabama, a turning point in the US civil rights movement, and the release of the King-centered movie, "Selma."

Coretta Scott King held on to the letter until 2003, then gave it to singer and social activist Harry Belafonte. She died in 2006.

When Belafonte tried to auction it in 2008, King's children objected, and the sale was cancelled. The two sides became embroiled in a legal battle.

A 2014 settlement allowed Belafonte to keep the letter and Belafonte gave it to his half-sister, Shirley Cooks. She and her husband, Stoney Cooks, a staff member of King's Southern Christian Leadership Conference, put it up for sale with other memorabilia.

Stoney Cooks said the letter was remarkable because Johnson, who had signed landmark civil rights legislation into law, wrote it while grappling with the unrest unleashed by King's murder.

"I thought that quick response showed something about the nature of the relationship between the two men," Cooks said.

Sixteen MLK items in all sold for a total of $99,668.

King's children Martin Luther King III, Dexter and Bernice have been involved in other court cases, sometimes against each other. They are currently involved in a legal case about whether his travelling Bible and Nobel Peace Prize should be sold.

Additional reporting by Reuters.