They were the murders that ended the 1960s, the decade of love, in a bloodbath that shocked the world.

Now, on the 40th anniversary of the killing of Hollywood actress Sharon Tate and her friends by Charles Manson's "Family", the gang member whose testimony convicted the killers has revealed for the first time her full involvement in the crimes.

On the night of August 9, 1969, Linda Kasabian was sent by Manson with three other members of his Family - Tex Watson, Susan Atkins and Patricia "Katie" Krenwinkel - to break into Tate's home.

There they tied up the actress and her friends and stabbed them to death. Kasabian acted as look-out.

Tate was married to the film director Roman Polanski and was eight months' pregnant. In the interview with Kasabian, for a new docu-drama called Manson, she tells how Tate pleaded for the life of her unborn child. She was stabbed 16 times. Her killers wrote the word "pig" in her blood on the wall.

"I saw a woman in a white dress and she had blood all over her and she was screaming and she was calling for her mom. I saw Katie stabbing her," says Kasabian, who is now 60.

The killings horrified America and the rest of the world and the subject has continued to fascinate ever since.

In the end, Manson was convicted of the murders of nine people, thanks to Kasabian who was the prosecution's star witness at his trial in 1970.

Kasabian had been living in hiding, under an assumed name, since the trial.

"I could never accept the fact that I was not punished for my involvement," says Kasabian. "I felt then what I feel now, always and forever, that it was a waste of life that had no reason, no rhyme."

Kasabian was a 20-year-old hippy with a 16-month daughter in July 1969 when she met members of Manson's Family and was asked to join their commune at a dilapidated ranch known as Spahn's.

There she met Manson, a 32-year-old racist who had already spent more than half his life in jail. About 20 people were living on the ranch, maintained by a life of petty crime and selling drugs.

"Manson - who was uneducated but highly intelligent - had this phenomenal ability to gain control over other people and get them to do terrible things," said Vincent Bugliosi, the lawyer who prosecuted Manson.

Armageddon was coming, Manson claimed as part of his racist, anti-establishment gospel that predicted a black uprising against the state.

He named this insurrection Helter Skelter because he believed details of it were revealed in the song of the same name by the Beatles. Manson was a talented musician who had met Beach Boys drummer Dennis Wilson.

The pair had recorded a few tracks, including one jointly written song Never Learn Not to Love that was subsequently recorded by the Beach Boys.

Manson also tried to set up a record contract with the producer Terry Melcher, but the deal had fallen through. Manson was angry with Melcher for not pursuing the deal and arrived at the latter's house at 10050 Cielo Drive to confront him.

However, Melcher had moved on and the house was now occupied by Roman Polanski and Sharon Tate. Manson was told to leave.

By now, Manson's control over his Family was virtually total. On July 25, he ordered three of them to go to the house of a drug-dealing acquaintance, Gary Hinman, to demand money.

Hinman refused, so they stabbed him to death, using his blood to paint the words "political piggy" on the wall - a grim rehearsal for what would occur at Tate's house.

Then Manson ordered Kasabian, Watson, Atkins and Krenwinkel to drive to Cielo Drive. "I felt excited, special, chosen," recalls Kasabian. "When we arrived at the Tate residence there were lights on the outside, the driveway was lit up. Tex got a rope and wire cutters and cut the telephone wires. There was a car coming so we got down. Tex jumped out and shot the gun four times. He told me to take the wallet from the kid he had shot. I got in the car. There was this person slumped over. I didn't see any blood or anything but I knew he wasn't there."

The others went inside the house. Polanski was in Europe, but Tate was entertaining three friends.

"You are all going to die," Watson told them after tying them with rope.

All four victims were stabbed to death. Kasabian took the weapons, wiped them clean and dropped them in a ravine.

The next day Manson sent his Family out again to kill and this time, at random, he selected the house of Leno and Rosemary LaBianca, wealthy owners of a chain of grocery stores. Manson broke in and tied them up. Then he left, ordering three of the Family to stab them to death.

Says Bugliosi: "The very name Manson has now become a metaphor for evil as a result. Manson was different from all other mass murderers. He got others to do his work and he was intelligent and manipulative. Most deranged cult leaders end up getting their followers to commit suicide en masse. Manson got them to carry out mass murders. That is why we remember him."

Nick Godwin, the film-maker behind the new docu-drama Manson, had only an assumed name to go on when his search began for Linda Kasabian.

"We also had a vague area, somewhere in the west of America, in which she was said to be living," he said.

She was found living in near-poverty in a trailer. Initially she refused to co-operate.

"None of her friends or neighbours knew about her dramatic past," said Godwin.

It took six months to establish a rapport and to get Linda to tell the story of the four weeks she lived with the Manson Family. She also had no idea that a British band had been named after her.

So Godwin's company Cineflix gave her a CD by Kasabian to listen to. She was pleasantly surprised, said Godwin.

As to Linda's role in the conviction of Manson, the prosecutor at the trial, Vincent Bugliosi, is in no doubt.

"She stood in the witness box for 17 or 18 days and never broke down, despite the incredible pressure she was under. I doubt we would have convicted Manson without her."

Manson is screening on Prime this Friday at 8.35pm.