That's the chilling confession Bobby Kennedy's nephew Michael Skakel gave to a frightened roommate about the brutal murder of pretty teen Martha Moxley 25 years ago. MICHAEL SKAKEL: New bombshell revelations.

And for the first time, The ENQUIRER can reveal the horrifying details of the slaying Skakel divulged to bunk-mate Harry Kranick, who spent 18 months with Skakel in a school for troubled youngsters after the murder.

"A girl was murdered 25 years ago and justice needs to be served," Kranick, now a 39-year- old businessman, told The ENQUIRER.

Kranick testified before the grand jury that just indicted Skakel.

On the night before Halloween, 1975, 15-year-old Martha Moxley and two friends were hanging out at the palatial home of Rushton Skakel, brother of Bobby Kennedy's wife Ethel, in an exclusive enclave of Greenwich, Conn.

After her friends departed, Martha was seen with Michael Skakel, then 15, and his 17-year- old brother Tommy.

Martha never returned home to her 14-room house across the street from the Skakel residence.

The next day, her body was found under a pine tree on her estate. She'd been bludgeoned to death with a golf club and dragged under the tree.

Her jeans had been pulled down around her ankles. The golf club -- belonging to the Skakel family -- was found in three pieces next to the body.

Amid talk of a Kennedy family cover-up, Tommy was shipped off to Ireland a month after the investigation began.

Michael, who was known to enjoy slaughtering animals, refused to cooperate with police.

For nearly a quarter-century, the case remained unsolved.

But finally, Harry Kranick's secret grand jury testimony has emerged as the key behind Michael's January 19 arrest on charges stemming from the murder.

"For years, Harry has been saying, 'I know who killed that girl -- he confessed to me,' " Kranick's longtime friend and former business associate Jeff Donahue told The ENQUIRER in an exclusive interview.

"Eighteen months ago he started talking to the grand jury and he told me, 'I'm telling them exactly what I've been telling you all these years.' "

The ENQUIRER met with Kranick three times in addition to interviewing Donahue.

Here's how Michael Skakel described the night of the brutal murder to roommate Kranick, according to Donahue.

Michael, Tommy, Martha and a few friends had been partying and were high on marijuana. The friends left, leaving Michael, Tommy and Martha alone.

Then Tommy left, and Michael and Martha started playing with a golf club, hitting a ball around.

Michael tried to make a sexual advance on Martha. She rebuffed him. She bent down to pick up the golf ball -- and Michael cracked her on the head with the golf club.

Said Kranick: "Michael told me, 'I went into a rage, totally losing control, and kept pounding Martha with the club as she lay on the ground.'

"He said he got the same kind of feelings he got when he was brutalizing animals.

"He told me, 'It got me excited.' He said in his mind he'd reached the pinnacle of power over others.

"As Martha lay unconscious, he pulled down her underpants and masturbated.

"Finally he shoved the broken golf club through the side of her neck."

Said Donahue: "It was after that stunning confession that Skakel told Harry, 'I killed that chick -- and I could kill you just as easily.'

"Harry admitted he was terrified. 'JUSTICE NEEDS TO BE SERVED' -- Michael Skakel's former roommate Harry Kranick.

"Harry also told me Michael said Ethel spoke to the Skakel kids, then spent a long time behind closed doors with Skakel's father.

"She was furious about how the murder was going to impact the family politically.

"Skakel bragged that the golf club was wiped clean of fingerprints . . . and he'd gotten away with murder."

A biography of Ethel quoted George Terrien, husband of Ethel's sister Georgeann: "Ethel was a nervous wreck. Georgeann told me later that Ethel said she was going to call Ted right away.

She also called her other advisers. She said, 'We can't let this touch the Kennedys.'"

Said Donahue: "Harry and I couldn't understand why the police never arrested Skakel -- we just assumed that it was Kennedy money and influence that was keeping him immune."

Kranick got close to Skakel when they roomed together at the Elan School in Maine, a radical treatment center for disturbed adolescents. Michael was sent there in 1978 after a drunk- driving incident.

"They shared a two-bunk room the size of a prison cell," said Donahue, a 35-year-old software developer who has known Kranick since he was 14.

"Elan was the last stop for some real bad kids. A concentration camp and hellhole, Harry called it.

"Kids who screwed up were paddled by other kids, and forced to fight each other in a boxing ring.

"At one time Harry and Skakel were assigned to clean a dirty dumpster with nothing but toothbrushes!

"Harry told me, 'Michael started telling me about how he got a kick out of killing animals.

He described how he put fireworks up dogs' behinds, blowing the dogs up and leaving them dead or maimed.

" 'He shot squirrels, and another time he said he buried a cat alive until only its head was above ground. Then he drove a lawn mower over it, decapitating it.' "

One night in 1978, Kranick told friends, Skakel tried to escape from the Elan facility.

He was caught -- and roommate Kranick was punished for not telling authorities of his friend's escape plan. Kranick was made to sleep in the dumpster he and Skakel had cleaned with toothbrushes, wearing nothing but adult diapers.

Skakel was hauled before a disciplinary meeting at Elan -- at which "a murder was discussed," according to a published report.

"Skakel was furious -- he realized Harry had ratted on him," said Donahue.

In another meeting at the school, said Donahue, "Harry told the story of Michael's confession. Michael at first denied it, according to Harry, but then broke down and sobbed, 'I killed her . . . I killed her.' "

But amazingly, Skakel's confession was never reported to police.

Shortly afterward, Kranick was allowed to leave Elan and return home to Chicago.

Kranick told The ENQUIRER he wouldn't repeat the revelations he'd made before the grand jury. But he admitted:

"You know an awful lot."

And two other close friends of Kranick confirmed to The ENQUIRER that he'd also told them details of the murder over a period of many years.

One of the friends, former tax attorney Eric Karr, told The ENQUIRER:

"Harry spilled the beans about Skakel when he came back from Elan. He said Michael was a nut and that he'd confessed, but nothing was going to happen to him.

"About two years ago Harry told me they were reopening the case. I thought 'fat chance' after all these years. But Harry turned out to be right!"