A new witness has come forward with startling new evidence in the case of Kennedy cousin Michael Skakel — and it could turn the case against the convicted killer upsidedown, The ENQUIRER has learned exclusively.

Skakel was convicted four years ago of killing 15-year-old Martha Moxley with a golf club in 1975. He was sentenced to 20 years to life, and is now appealing his conviction to the Supreme Court. His fate may rest in the hands of Fred Sibley, who says he heard Martha’s dying screams at a time when Skakel claims to have an alibi. Sibley also insists there were others involved in her death!

Sibley was a 20-year-old busboy working at the Homestead Inn in Greenwich, Conn., taking out the trash at about 9:45 p.m. on Oct. 30, 1975 — the night of the vicious slaying. “I heard a girl screaming, incredible screams, coming from the direction of the Moxley home,” Sibley told The ENQUIRER.

“Why won’t one of you help me? Why?'” she screamed. “Then she stopped, and dogs began barking.” Sibley went back inside and, a few minutes later, left to walk home.

“By the entrance, a black Lincoln Continental came flying around the curve out of control, actually up on two wheels, and nearly hit me!”

Angry, Sibley made an obscene gesture at the car, which screeched to a halt. The rear door opened and a man started to jump out. “Someone inside screamed: ‘We’ve got to get out of here!'” Sibley recalled. “There were at least four people in the car, but it was too dark for me to recognize anyone.”

Sibley’s story of the screams would place Martha’s murder at a time when Skakel claimed to have been watching television with family members.

Sibley says his parents convinced him to keep quiet, fearing that his politically active father would face a Kennedy backlash.

For more on this story, pick up this week’s issue of The ENQUIRER.