A nephew of the late Robert F. Kennedy was charged with murder today in the beating death of a 15-year-old girl nearly a quarter century ago, a widely chronicled case that is part of the annals of Kennedy family troubles.

Michael Skakel turned himself in to Greenwich, Conn., police this afternoon in response to an arrest warrant, the first charge ever lodged in the mystifying death of Martha Moxley. His attorney, Michael Sherman, of Stamford, Conn., said Skakel was released on $500,000 bond.

"His plea will be not guilty," Sherman said. "He has consistently maintained his innocence."

Prosecutors reportedly claim they have witnesses who testified before a grand jury that Skakel told them of his crime.

Because Skakel was 15 at the time of the crime, it was unclear whether the case will be handled in juvenile or adult court.

Skakel, now 39, is charged with beating Moxley to death on Halloween eve 1975, in the gated Greenwich enclave called Belle Haven, where they lived on neighboring estates. Moxley was battered about the head so hard with a 6-iron golf club that it splintered into shards, one of which was used to stab her through the neck. Part of the alleged murder weapon was traced to a set of golf clubs at the Skakel home.

Residing at the Skakel home then were three young men who for years would be considered as potential suspects: Michael Skakel, 15; his brother, Thomas Skakel, 17; and Kenneth Littleton, a live-in tutor for the Skakel children.

Skakel is the son of wealthy industrialist Rushton Skakel, who is the brother of Robert F. Kennedy's widow Ethel Kennedy. Michael Skakel's ties to money, power and politics have spawned conspiracy theories about who committed the homicide and why it has remained unsolved for so long.

New evidence came forward in 1998, which prompted the appointment of a one-member grand jury, as is Connecticut's practice, and today's charge is based on that grand jury investigation.

Littleton testified before the grand jury in 1998 in exchange for immunity from prosecution. His statements were characterized in news reports at the time as potential breakthroughs in the case, but his attorney, Eugene Riccio, of Bridgeport, has refused to discuss their content.

"I think cases like this rip apart a number of lives in a number of different directions. To say cataclysmic would be an understatement," Riccio said today.

Though not part of the direct Kennedy family line, Skakel has figured in other family troubles of the Kennedy progeny, such as the 1991 rape case against William Kennedy Smith. In fact, the Moxley homicide investigation--then dormant--was reopened as a result of rumors that Smith was at the Skakel estate the night of the Moxley slaying. That rumor was never corroborated.

In 1997, Skakel was entangled in another Kennedy family scandal when he testified about the allegations that a cousin, the late Michael Kennedy, for whom Skakel was a driver, was having an affair with a teenaged baby sitter.

The murder case against Skakel apparently is based on statements he made to fellow students while in a substance abuse treatment and educational program in the late 1970s, according to the Associated Press.

Skakel was named as the prime suspect in the case in a 1998 book, "Murder in Greenwich," by former Los Angeles police detective Mark Fuhrman, whose racist statements became one of the many sideshows of the O.J. Simpson murder trial.

Fuhrman said in a brief interview today that he saw a book proposal of Skakel's in which Skakel alludes to his role in the murder. Fuhrman said the proposal reveals Skakel's obsession with Moxley and jealousy at the attentions she gave his brother. Shortly after the killing, most police attention focused on Thomas Skakel, who was reportedly the last to see Moxley alive. Friends, in statements to police, said they saw the two horsing around on the lawn and flirting.

"It's your basic semi-love triangle," Fuhrman said. "Obsession, rage, jealousy, substance abuse all at the same time."

CAPTION: Michael Skakel enters Greenwich Police Department to surrender on charge of murdering Moxley when he was 15. He was freed on bond.

CAPTION: In 1975, Martha Moxley, 15, was found beaten to death with a 6-iron golf club in Connecticut.