The assassination was over in a few seconds. In the photograph of that moment, Bobby Kennedy, his eyes open and glazed, lies on his back on a hotel pantry floor, his head cradled by a busboy dressed starkly in white - a tableau that seems almost angelic were it not so brutal.

Less than 26 hours after being shot early on June 5, 1968, right after winning the California presidential primary, Kennedy was dead. He was 42.

Three major assassinations rocked America in the 1960s. Two of the assassins - Lee Harvey Oswald, the killer of John F. Kennedy, and James Earl Ray, who shot Martin Luther King Jr. - are dead. But Sirhan Sirhan, convicted of killing Robert F. Kennedy 40 years ago this week in the Ambassador Hotel in Los Angeles, is living out his days in the California state prison at Corcoran. He is 64 and has never fully explained what happened that night other than to say he can't remember it.

Sirhan was a seemingly unremarkable man. He was a Palestinian who was raised in the Middle East until he was 12, when his family settled in Southern California. Before the Kennedy assassination, he held a series of menial jobs and at one point worked at the Santa Anita racetrack and had hoped to be a jockey.

After Los Angeles police found his diary, in which he had written, "RFK must die," investigators concluded that he was angry about Kennedy's support for Israel and somehow had tied the assassination date - he wrote that Kennedy must be killed "before 5 June 68" - to the one-year anniversary of the Six-Day War.

Open and shut

Los Angeles police, who declined Monday to comment on their investigation, deemed the assassination an open-and-shut case - Sirhan did it by himself. Independent investigators who have looked at the case over the years, however, suggest otherwise.

"The interesting thing is how under-examined the Robert Kennedy assassination is, compared to President Kennedy and Martin Luther King Jr.," said David Talbot of San Francisco, author of "Brothers," a book that looks into Robert Kennedy's own investigation into his brother's death and his conviction that JFK was the victim of a conspiracy.

"Bobby remains the unknown territory," Talbot said. "But even if you look at it minimally, there are questions that come to mind."

Among them:

-- Sirhan fired his .22-caliber revolver from a few feet in front of Kennedy, according to police, yet Los Angeles County coroner Thomas Noguchi reported that the fatal shot was fired less than one inch from Kennedy's head, behind his right ear. Of the four shots fired at Kennedy, all came from the rear. None of this was raised at Sirhan's trial because his defense was based on the theory that he suffered from "diminished capacity" rather than on any challenge of prosecutors' evidence.

-- Sirhan's revolver held eight rounds; a radio reporter's tape recording of the shooting has sounds of what one audio expert describes as 13 shots. Sirhan never had a chance to reload before bystanders tackled him. Two of the sounds on the tape are what forensic experts call "double shots," which means two shots so close together that they couldn't have come from the same revolver.

-- Several witnesses saw a security guard just behind Kennedy draw his revolver, and one reported seeing him fire it.

-- Over the years, Sirhan has told investigators who interviewed him in prison that he was in a hypnotic trance during the shooting and can't remember it at all. He said he could not remember writing, "RFK must die." He did not respond to an interview request for this story.

Night of celebration

On the night Kennedy was killed, the hotel ballroom was filled with supporters celebrating his victory in the California primary and looking to the Democratic convention in Chicago. The last thing Kennedy said from the ballroom podium, just after midnight, was, "My thanks to all of you, and now it's on to Chicago, and let's win there."

In the pantry, as Kennedy moved through the crowd, he was surrounded by friends, including Paul Schrade of the United Auto Workers, labor chairman for Kennedy's campaign.

"All of a sudden, I got hit in the head by a bullet," Schrade said. "I shook violently. I thought I was being electrocuted. When I came to, I was on the floor."

Schrade was one of five people besides Kennedy who were hit by bullets. For the past 33 years, he has been investigating the shooting.

Mystery bullet hole

Unlike the JFK assassination, which created an outdoor crime scene in Dallas sprawling from the grassy knoll to the Texas School Book Depository, the shooting of Robert Kennedy happened in a confined space. Stray bullets ended up buried in walls and the ceiling, where they could be tracked down.

In photos, police investigators can be seen circling what they later said was a bullet hole in a ceiling panel, behind where Sirhan fired. For Sirhan to have shot into that panel, he would have had to "either turn around or the bullet would have to have made a U-turn," said Philip Van Praag, a retired electrical engineer and audio expert who co-authored a book about the case.

Then there was the mystery of the woman in the polka dot dress. According to witness Sandra Serrano, the woman fled from the hotel kitchen with an unidentified man, shouting, "We shot him, we shot him." When a bystander asked who got shot, the woman said, "We shot Kennedy." Other witnesses reported seeing the woman, though it is not clear whether they heard the comment.

In a new film about the assassination, "RFK Must Die," Irish documentary maker Shane O'Sullivan asked Serrano about what happened later. She said Los Angeles police spent hours trying to convince her she was wrong in what she saw, and she finally gave in. Forty years later, however, she told O'Sullivan that her original version was correct.

'I don't remember'

In fact, the iconic polka dot dress is also something fixed in the mind of Sirhan himself.

William Turner, a retired FBI agent who wrote a book about the case, says he interviewed Sirhan in prison in 1975.

"He told me, 'I don't remember anything after the woman in the polka dot dress asked me for coffee, and heavy on the cream and sugar,' " said Turner, who lives in San Rafael. "He said he had amnesia from that time until he was overpowered in the pantry after the shots were fired. He said, 'I must have done it, but I don't remember.' "

Turner thinks Sirhan was "hypno-programmed to shoot" and that he was a real-life Manchurian Candidate - the fictional brainwashed dupe whose controllers want to assassinate a presidential candidate. Turner suspects the same villains as do the JFK conspiracy theorists - "organized crime and, predominantly, people from the CIA."

Van Praag and a fellow investigator, former American Academy of Forensic Scientists president Robert Joling, don't subscribe to any one conspiracy theory, but they are convinced more than one gunman was involved. The two have written a book about the killing, whose title, "An Open and Shut Case," is a dig at the police investigation.

Van Praag, a former senior instructor in commercial audio-video systems for Ampex Corp., analyzed a tape recording of the killing made by a Polish radio reporter. He said he heard 13 shots over five seconds and was able to isolate the sounds well enough to say that two different weapons were firing during those five seconds.

Guard passed polygraph

One of those weapons, according to the documentary, "Conspiracy Test: The RFK Assassination," which ran on the Discovery Times Channel a year ago, could have been held by Thane Eugene Cesar, the security guard who was near Kennedy.

Dan Moldea, who wrote a book, "The Killing of Robert F. Kennedy: An Investigation of Motive, Means and Opportunity," said he thought for years that "Cesar had done it." But in 1987 he persuaded Cesar to undergo a polygraph examination that the former guard "passed with flying colors," Moldea said.

"He's being accused of murder all over the place," Moldea said, adding that he is now Cesar's protector and would be willing to "bring him forward" if authorities ever reopen the case.

In fact, reopening the case is not a far-fetched idea.

Joling says an "independent panel of forensic scientists" should be created to "reinvestigate this matter on all the evidence." The case "should be resolved in a truthful, factual and honest presentation," he said.

"Let the chips fall where they may. That way, at least, the American people will know that somebody without a stake in the outcome made this finding."