DALLAS — Forty years after John F. Kennedy's assassination, an overwhelming majority of Americans do not believe the official conclusion that a loser named Lee Harvey Oswald, acting alone, killed the president with a cheap mail-order rifle fired from the Texas School Book Depository.

Thousands of books, movies and Internet chatrooms have fueled dozens of conspiracy theories that it was a plot by the Mafia, the Cubans, the KGB, the CIA, even Vice President Lyndon B. Johnson, and that other shots came from the grassy knoll or other spots around Dealey Plaza.

Despite four decades of technical improvements in forensics and film enhancement, the questions at the heart of the theories have changed little since Nov. 22, 1963: Who killed Kennedy as he rode in an open Lincoln convertible through downtown Dallas? How many shots were fired? Did Oswald have help?

Oswald, arrested shortly after the assassination, was silenced two days later when nightclub owner Jack Ruby gunned him down as police transferred him from a jail. Answering the question of who was behind the shooting was left to the government-appointed Warren Commission, which after a 10-month investigation concluded in 1964 that Oswald acted alone, firing from the book depository's sixth floor.

Yet, today only 32 percent of American adults accept that finding, according to an ABC News poll. The poll, conducted earlier this month, found that 70 percent think the assassination was part of a broader plot; 51 percent believe there was a second gunman; and more than two-thirds believe there was a government cover-up.

In 1966, three years after Kennedy's death, 46 percent of people surveyed in a Harris poll believed the assassination was part of a broader plot. By 1983, that number had reached 80 percent in an ABC poll.

Some experts have suggested that the Vietnam War and Watergate deepened Americans' cynicism and eroded trust in government.

"Many people look at the Kennedy assassination as a turning point, when people started realizing and thinking and believing their government would lie to them and lie to them repeatedly," said Gary Mack, curator of the Sixth Floor Museum at Dealey Plaza in Dallas.

Perhaps the most persistently questioned finding of the Warren Commission is the "magic bullet" theory.

The theory assumes that Oswald alone fired three shots and that one bullet zigzagged through both Kennedy and Texas Gov. John Connally. The bullet is said to have gone through Kennedy's throat, then into Connally, puncturing his lung, hitting his rib and wrist and then exiting relatively unscathed.

Some historians, forensic experts and conspiracy theorists do not buy it.