MONTGOMERY, Alabama (CNN) -- From the time he first emerged as a civil rights leader, the Rev. Martin Luther King Jr. lived with the threat of death, but he never wavered in his commitment to non-violence.
Dr. Martin Luther King Jr. believed the cause they were fighting for was worth dying for.
"Dr. King made it rather clear that the cause that we were fighting for was not only worth living for, but it was worth dying for, if need be," said Fred Gray, the lawyer who helped King lead the fight to desegregate city buses in Montgomery, Alabama, in 1956.
A month after blacks began a bus boycott, a midnight caller warned King that he would be sorry he ever came to Montgomery. Three days later, his house was bombed.
Angry blacks gathered outside King's home, but Gray said, "Once he found out his family was safe and secure, he simply went out, talked to the crowd, and told them to go home, and they went."
King knew what could happen when he led demonstrations in Birmingham, Alabama, in 1963, facing fire hoses and police dogs in an effort to desegregate downtown businesses.
Longtime aide Andrew Young said, "Going to Birmingham was to him the possibility of an imminent death."
Another aide, the Rev. Wyatt Tee Walker, said when he kissed his own wife and children goodbye to go there, "I thought I would never see them again. I didn't think I would come out of Birmingham alive. I didn't think King would."
But in September, after the city had yielded, it would be four young girls who would be killed in the bombing of the 16th Street Baptist Church.
King felt responsible, Young said. "Most of those days he was in a deep depression."
When King delivered the eulogy for the victims, lawyer Clarence Jones said, "It was one of the few vivid times ... where I observed tears, him crying as he was speaking."
Yet King told the mourners, "In spite of the darkness of this hour ... we must not become bitter, nor must we harbor the desire to retaliate with violence."
King flew into Memphis, Tennessee, the first week of April 1968 to try to lead a peaceful march in support of a strike by black garbage workers. On a stormy night, he spoke at a church rally.
"He talked about death more than I heard him talk about it at any one given," Memphis NAACP leader Rev. Billy Kyles said.
King told the crowded church, "I don't know what will happen now. We've got some difficult days ahead. But it really doesn't matter with me now. Because I've been to the mountaintop. And I don't mind."
Young said the speech was one that King had made before when times were dangerous.
"Because he had done it before, and we'd gone on to the next place, I wasn't really taking it seriously," Young said. "It was just a great speech, but I never thought I was listening to his last speech."
King ended: "I'm not fearing any man. Mine eyes have seen the glory of the coming of the Lord."
"We were just boo-hooing. And we had to actually help him to his seat," Kyles said. "He gave it his all, and somehow, I guess he knew that would be his last hurrah."
The next evening, King walked out of his motel room to have dinner at Kyles' home.
"I said, 'Guys, come on, let's go,' and I walked to the stairs," Kyles remembered. "Before I could get to the stairs, the shot rang out. 'Ka-POW!'
"Blood was everywhere," Kyles said. "I took a spread from one of the beds and covered him from the neck down. He never said a word."