"There's still so much sentiment for John F. Kennedy, and so much of it is colored by the assassination," said David Crockett, a political scientist at Trinity University in San Antonio. "He's the young, attractive, tragic martyr figure assassinated on television, with a wife who's mourning."

When many Americans close their eyes, they can still see Kennedy's 3-year-old son, "John John," bravely saluting his father's flag-draped coffin.

After a 10-month investigation, the Warren Commission in 1964 concluded that Lee Harvey Oswald alone killed Kennedy, firing shots from the Texas School Book Depository's sixth floor.

Doubts lingered, however, and in 1978, Congress impaneled a committee to again investigate the assassination. The panel largely relied on the recording of a police motorcyclist's microphone.

The committee's conclusion: Four shots were fired, with one coming from a grassy knoll downtown. In other words, it concluded, Oswald didn't act alone.

But after further studies, the Justice Department in 1988 concluded there was no "persuasive evidence" of conspiracy and formally closed the investigation.

Oswald was killed two days after Kennedy's assassination — gunned down by Dallas nightclub owner Jack Ruby as he was transferred from one jail to another.

A Dallas jury convicted Ruby of murder in 1964 and sentenced him to die. An appellate court ruling later set the verdict aside, and Ruby died of cancer in prison in 1967 before he could be retried.

Kennedy's daughter, Caroline Kennedy Schlossberg, who was four days shy of her 6th birthday when her father died, is the sole survivor of her immediate family. Jacqueline Kennedy Onassis died of cancer in 1994 and John F. Kennedy Jr. died along with his wife and sister-in-law in the 1999 crash of a small plane he was piloting.

The crash brought still more pain to a family that dealt first with Kennedy's slaying, then with the assassination of his brother, Robert, during his 1968 presidential campaign.

All of which help explain the unending interest in all things Kennedy.

"They've just had great triumph and great tragedy," said Patrick Maney, a presidential historian at the University of South Carolina.

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In Dallas itself, the anguish for some still seems as fresh as on that Friday afternoon 40 years ago.

"There are people who lived in Dallas in '63 who will never come to this site. It is too painful," said Jeff West, executive director of The Sixth Floor Museum, which chronicles Kennedy's life, death and the era in which he lived.

But for others, acknowledging Dallas' place in history helped the healing. "There are people who were here in '63 who are very proud and pleased that we did something to commemorate and mark the spot," West said.