STAMFORD, Conn., Aug. 17 -- A juvenile court judge ruled today that Michael Skakel should be tried in the 1975 slaying of Martha Moxley, finding that there was "reasonable cause" to believe that Mr. Skakel, a nephew of Ethel Kennedy's, bludgeoned the 15-year-old girl to death when they were neighbors in Greenwich, Conn.

The judge, Maureen Dennis, must now make the crucial decision of whether Mr. Skakel should be tried as an adult. Mr. Skakel, who is now 39, was charged as a juvenile because he was 15 at the time of Miss Moxley's death. If convicted as an adult, he could face a sentence of life in prison; conviction as a juvenile would carry a maximum sentence of four years and could very well involve no jail time at all.

The Moxley case -- because of the suspected involvement of a Kennedy relative, its setting in one of America's wealthiest suburbs, and the nearly quarter-century delay by the authorities in obtaining an indictment -- has been the sensational subject of numerous best-sellers and television documentaries. It also set judicial precedent in Connecticut.

Under state law, as it existed in 1975, pertaining to juvenile crimes, Judge Dennis was required to conduct a hearing to determine whether there was reasonable cause to proceed with the case against Mr. Skakel and, if so, whether the case should remain before her in juvenile court or be transferred to State Superior Court and heard by a jury that could convict him as an adult.

During the hearing in June, two former classmates of Mr. Skakel at a school for troubled youths in Maine testified that they had heard him in the late 1970's confess to killing Miss Moxley. In her 13-page decision today, Judge Dennis wrote that she found those witnesses "credible." She also found credible a witness who said Mr. Skakel once admitted being at the murder scene on the night of Miss Moxley's death.

Judge Dennis wrote: "Having considered all of the state's documentary, photographic and testimonial evidence, as well as the respondent's submissions for purposes of rebuttal, the court finds that there is reasonable cause to believe that the respondent has committed murder." She ordered an "investigation" into whether Mr. Skakel should be tried as an adult in Superior Court.

Legal experts said that investigation was a procedural matter to be conducted by the juvenile court system. Under normal circumstances, they said, it would be used to determine a juvenile defendant's family situation and performance in school, any previous criminal record and other pertinent information that could be used in deciding whether the juvenile or adult court system was the more appropriate jurisdiction.

"Nothing about this case is normal or usual,' said Daniel P. Weiner, a veteran criminal defense lawyer, who has represented many juvenile clients. Mr. Weiner said that Connecticut's juvenile court system had always emphasized rehabilitation and that if Mr. Skakel had been convicted as a teenager, the court's inclination at the time would likely have been to send him to reform school.

Now, of course, that is not an option. "You have a 39-year-old man," Mr. Weiner said. "How can you send him to bad-boy school? That's preposterous."

The situation is unprecedented for Connecticut, leaving Judge Dennis with a perplexing judicial challenge: how to allow for an appropriate sentence if Mr. Skakel was convicted while still affording him all of the rights he would have been entitled to as a juvenile.

The assistant state's attorney, Jonathan C. Benedict, and his leading investigator on the case, Frank Garr, praised the judge's finding of reasonable cause and said they expected her to transfer the case to Superior Court. "This will eventually be transferred to the adult court," Mr. Garr said, adding that he expected a decision shortly. "I feel pretty confident at this point."

Miss Moxley's brother, John, said the family was pleased with the decision but said they wanted the case to be transferred. "We are very pleased that Judge Dennis verified the grand jury findings," he said in a television interview on WTNH. "I think we're a little disappointed that she did not go all the way and recommend the transfer."

Mr. Skakel's lawyer, Michael Sherman, said that he disagreed with the judge's assessment of the witnesses but that he was not surprised by her decision, even as he reiterated his client's innocence.

"I am neither surprised nor disappointed," he said. "I always believed this case would be tried, whether by a judge or a jury." He added, "I really do trust the system and I believe that in the end Michael will be exonerated."

Miss Moxley, a pretty and popular 15-year-old, was found with her skull bashed in, lying under a tall pine tree in the yard of her family's home in the wealthy Belle Haven enclave of Greenwich on Oct. 31, 1975. Investigators found pieces of the murder weapon, a Tony Penna six-iron golf club that was later determined to have come from a set owned by Mr. Skakel's father.

Gregory Coleman, who attended the Elan School in Poland Spring, Me., with Mr. Skakel in the late 1970's, testified at the reasonable-cause hearing that Mr. Skakel told him that he had made romantic advances toward Miss Moxley and that when she rebuffed him, he "drove her skull in" with a golf club.

Mr. Coleman also testified that Mr. Skakel said: "I am going to get away with murder. I am a Kennedy."

In her decision, Judge Dennis wrote, "From the evidence presented, a reasonable and prudent person of caution could logically infer" that Mr. Skakel "retrieved the Tony Penna number 6 iron golf club, the instrument used to murder the victim from his family home and/or the outbuilding thereon and ran through the woods and pine trees near his home to the Moxley property."

"A person of reasonable caution could further infer and deduct," Judge Dennis continued, that Mr. Skakel "was at the location where the murder took place and at a time when it occurred." She noted that Mr. Skakel "has acknowledged to at least two individuals that he was at the scene where the body was found."

The judge also pointed to testimony by Mr. Coleman and another witness, Andrew Pugh, who was a childhood friend of Mr. Skakel in Greenwich. Mr. Coleman and Mr. Pugh both testified that Mr. Skakel told them he masturbated at the murder scene. "It is also more than mere coincidence," Judge Dennis wrote, "the respondent made statements concerning his masturbating either on the victim, or in the tree on the Moxley property on the night the victim was murdered."