If you’re going to make liberal use of a voice-over narration, it helps not to have a voice that makes people want to leave the room. Soledad O’Brien, who narrates a new CNN documentary on Martin Luther King Jr., airing Thursday, “Eyewitness to Murder: The King Assassination,” has a delivery that is by turns tone-deaf, hectoring, and steeped in clichéd TV-reporter emphases and intonations. Her task is to recount the events that led up to the death of Rev. King — 40 years ago this Friday — whom she calls “the greatest civil rights leader in American history,” “an American martyr,” and a follower of the philosophy of non-violent protest practiced by the Indian spiritual leader, Mohandas Gandhi. Yet far too much that comes out of her mouth sounds aggressive and mildly bullying.

This is bad news for CNN, not to mention its viewers, because much of “Eyewitness to Murder” is quite good, and it’s only the first in a series of three two-hour documentaries appearing under the general title, “Black in America,” which Ms. O’Brien will present. “The Black Man,” slated to air July 23, attempts to determine to what extent life has improved for African-American males during the last four decades by focusing on the graduates from the 1968 class of Little Rock Central High School. “The Black Woman & Family,” appearing the following night, will try to do something similar from a female perspective, measuring to what extent Rev. King’s hopes for the future have come true by relating the experiences of an African-American family in Houston. Between April and June, CNN will also be running shorter, weekly reports on such topics as black marriage rates, the toll of HIV/AIDS, and how black Americans now fare in terms of education, the professions, and life expectancy as compared to the rest of the population.

Given the very real possibility that a black man, Barack Obama, may become the country’s next president, the timing is excellent, even if the menu of topics seems a bit predictable.

“We had to change the world,” Rev. King’s closest aide, Andrew Young, tells Ms. O’Brien near the beginning of “Eyewitness to Murder” as we watch the familiar footage of Rev. King and his followers set upon by police dogs in 1963, attacked by Alabama state troopers in 1965, and having bottles and bricks hurled at them when they ventured north to Chicago. All the while, of course, they were monitored obsessively by FBI chief J. Edgar Hoover, who wiretapped Rev. King’s hotel rooms (with the cooperation of Attorney General Robert Kennedy) and taunted him as “a tomcat with degenerate sexual urges,” underscoring Mr. Young’s irrefutable point to such an extent it makes the claim sound modest.

Mr. Young is one of many people interviewed, including David Garrow, author of “The FBI and Martin Luther King, Jr.” John Lewis, the Democratic congressman for Georgia, and Jerry Ray, the brother of James Earl Ray, who was convicted of killing Rev. King. But Mr. Young has a unique way of humanizing a man who is in danger of becoming a bit too much of a saint. In one anecdote, he describes how Rev. King expected an assassination attempt, especially after the death of President Kennedy in 1963, but jokingly suggested that one of his inner circle would take the bullet for him. He then promised that, as a reward for such self-sacrifice, he would preach the greatest funeral sermon ever heard on the bullet-taker’s behalf.

Interestingly, Mr. Young remains skeptical of Rev. King’s decision to protest the war in Vietnam while simultaneously fighting for civil rights, and notes that any and all communication with President Johnson, who wrote civil rights legislation into law with Rev. King standing behind him, ceased immediately thereafter.

Although the cat-and-mouse aspect of the CNN documentary is complicated by doubts as to whether Ray really was the assassin, “Eyewitness” is at its most compelling when it traces the paths, particularly on Ray’s side, that led to the balcony at the Lorraine Motel in Memphis, Tenn., where Rev. King was shot on April 4, 1968. There is exhaustive analysis of the crime scene, as well as multiple interviews, including one with George Loenneke of the Memphis Fire Department — the only known living witness who saw the shot being fired — and provocative rumors about mysterious government agents who scouted a rooftop across from the motel shortly before the murder. There is also Ray himself, who managed to drive out of Memphis in a white Mustang without getting caught and, using an alias, made it as far as Lisbon before being apprehended. In an age of buzzing police helicopters, biometric eye-scanners, and omnipresent security cameras, this seems almost incredible. It’s also impossible not to be drawn in by Ray’s sojourns in seedy anonymous rooming houses (the one across from the Lorraine is now part of the National Civil Rights Museum), his attempts to filch passports from drunk sailors in Montreal, his supposed meeting with an enigmatic character named “Raoul” who may have set him up, and by his ghostly, strangely charismatic face.

“Eyewitness to Murder” is full of absorbing footage from a world that already looks and sounds antique, yet which continues to haunt us. There’s plenty here to hold your interest, particularly the host of alternative scenarios exploring the possibility that Ray did not kill Rev. King. (Most of Rev. King’s family and associates have either exonerated him of the crime or believe he was a patsy, and Ray himself recanted his confession two weeks after making it.) In the end, it all adds up to another tragically murky episode from the dark side of the 1960s, and murky is how it’s likely to remain.

bbernhard@earthlink.net