On the morning of April 3, 1968, Martin Luther King Jr. readied to head to Memphis, Tennessee, for the third time in as many weeks in support of the city's striking sanitation workers.

The first trip had brought a crowd of 15,000 to hear the weary but still commanding civil rights leader speak at Bishop Charles Mason Temple on March 18. He had returned to lead a march 10 days later, only to see the demonstration dissolve into rioting and chaos that left a high school student dead.

Dismayed by the outcome and discouraged by staffers who wanted him to focus on the upcoming "Poor People's Campaign" in the nation's capital, King nevertheless was determined to lead a second, successful march in Memphis to prove that his method of nonviolent demonstration still had teeth.

King was 'exhausted' during his last days

As recounted in the book Redemption: Martin Luther King Jr.'s Last 31 Hours, MLK's Memphis plans faced hurdles from the get-go, starting with the bomb threat that delayed his flight out of Atlanta, Georgia.

After arriving, King and his cohorts were slapped with an injunction that prevented them from leading a demonstration in the city. He huddled with his legal team at the Lorraine Motel to discuss strategy, and followed with another meeting with a local Black power group called the Invaders, aiming to head off a potential repeat of the riot-instigating actions that had torpedoed his last effort.

Fighting a cold, and exhausted after weeks of traveling, King sent his chief lieutenant, Ralph Abernathy, to speak on his behalf at Bishop Charles Mason Temple that evening. Although the stormy weather had depressed the turnout, Abernathy sensed the crowd's disappointment with King's absence and convinced the celebrated orator by phone to make an appearance.

He delivered his famed 'I've Been to the Mountaintop' speech one day before his death

In what became known as his "I've Been to the Mountaintop" speech, King took the audience on a time-traveling journey through the highlights of human civilization and revealed that here, amid the struggle for human rights in the second half of the 20th century, was exactly where he wanted to be. He then recalled how he had been stabbed 10 years earlier when a mere sneeze could have ruptured his aorta and prevented him from being part of civil rights history.

"I don't know what will happen now; we've got some difficult days ahead," he said, nearing the prescient climax. "But it really doesn't matter with me now, because I've been to the mountaintop. Like anybody, I would like to live a long life — longevity has its place. But I'm not concerned about that now. I just want to do God’s will. And so I'm happy tonight; I'm not worried about anything; I'm not fearing any man. Mine eyes have seen the glory of the coming of the Lord."

Utterly spent, King was helped back to his seat, tears welling in his eyes. However, the emotional speech was cathartic for a man who had endured so much stress. In the biography Bearing the Cross, Abernathy recalled how his friend seemed "happy and relaxed" at dinner after the rally, and King found the energy to stay up well into the night, joking around with the others.

Martin Luther King during his "I've Been to the Mountaintop" speech

Martin Luther King delivering his "I've Been to the Mountaintop" speech on April 3, 1968.

King was shot in the face on a motel balcony

After waking up late on April 4, King discussed organizational matters with his staffers, before hearing the good news: His lawyers had persuaded the judge to lift the injunction, allowing for a tightly controlled march on April 8.

At around 6 p.m., as he prepared for dinner with a local minister, King stepped out to the balcony of room 306 at the Lorraine to chat with colleagues waiting in the courtyard below. A gunshot suddenly pierced the air, and the others recovered from their momentary confusion to find King prone on the balcony, bleeding profusely from the right side of his face.

Although he was rushed off to St. Joseph's Hospital relatively quickly, the bullet had punctured several vital arteries, fractured his spine, and 39-year-old King was declared dead at 7:05 p.m.

READ MORE: 17 Inspiring Quotes by Martin Luther King Jr.

Riots and tributes followed his death

King's assassination led to the outbreak of violence across American cities, with reports of more than 40 casualties, another 3,500 injuries and 27,000 arrests accumulating over the next several days. But there were also noble tributes to the slain civil rights leader: President Lyndon B. Johnson decreed April 7 a day of national mourning, while opening day of the Major League Baseball season and the Academy Awards telecast were both suspended.

The April 8 march went through as planned, with the widowed Coretta Scott King leading an estimated 42,000 demonstrators through the Memphis streets. The following day, another 50,000 supporters accompanied King's mule-drawn casket through downtown Atlanta, to South-View Cemetery.

On April 16, the cause that had consumed the final days of King's life realized its goal, as the city of Memphis agreed to improved wages and the recognition of the sanitation workers' union.

James Earl Ray

James Earl Ray being led to his cell by Shelby County Sheriff William Morris on July 19, 1968.

James Earl Ray pleaded guilty, though conspiracy theories remain

Meanwhile, a massive manhunt led the FBI from Bessie Brewer's Rooming House, across from the Lorraine Motel, to California, Alabama, Canada, Portugal and finally to London's Heathrow Airport, where James Earl Ray was apprehended on June 8. He pleaded guilty the following March, earning a 99-year prison sentence, but almost immediately recanted the plea, insisting he was part of a larger conspiracy.

In a twist, many members of King's family and inner circle eventually went public their belief that Ray was not the killer. In 1999, the family won a wrongful death suit against Memphis cafe owner Loyd Jowers, who claimed to have hired the true assassin. This spurred the launch of a new investigation from the U.S. Department of Justice, which ultimately determined that there was no reason to reopen the case.

More than 50 years after King's final breath, the full story behind his killing remains unknown. Nevertheless, the tragedy cemented his place as an icon of the transformative era, forever frozen on the mountaintop as a man who lived out his life in the service of others.