"I have said that if the government wouldn't try Jeff, then I would kill the son of a bitch. And I would do it, in a minute."

For nine years Alfred Kassab and his wife, Mildred, have lived in grim self-imposed isolation. They don't see their old friends much anymore, and since they moved from Long Island to an adult community in Cranbury, N.J., they haven't bothered to make many new ones. The Kassabs, both 58, have never recovered from the shock of the savage triple murder that took the lives of their only daughter, Colette, and their two small grandchildren, Kimberly and Kristen.

It is not grief alone that has become the couple's obsession; it is their determination to exact retribution. The Kassabs are convinced they know the killer's identity. For four years Fred Kassab stubbornly pressured federal prosecutors to file murder charges against his former son-in-law, Dr. Jeffrey R. MacDonald, 35, now a physician in Long Beach, Calif. Then Kassab waited patiently while MacDonald appealed his indictment three times all the way to the U.S. Supreme Court. Finally, in mid-July, when MacDonald went on trial in Raleigh, N.C., what Kassab felt was not happiness but vindication. "I have been accused of running a vendetta," he says calmly. "I am."

A native Canadian, of Egyptian and Syrian descent, Kassab has long been a man to be reckoned with. Attached to British intelligence during World War II, he parachuted behind German lines on six occasions to make contact with the French Resistance. After the war he settled in New York, went to work for a land development company and married Mildred, a widowed mother of two (Colette and an older brother, Robert, now 38). "I don't think anyone was closer to Colette than I was," he says. "She would never let anyone refer to me as anything but her father."

When the family moved to Patchogue, L.I., Colette met Jeff MacDonald and dated him all through high school. "He was an all-American boy," Kassab remembers. "He could charm the birds out of the trees." Jeff and Colette married in 1963, while he was at Princeton and she at Skidmore. After earning his M.D. from Northwestern, MacDonald interned at Columbia-Presbyterian Medical Center in New York. Later he was commissioned in the Army Medical Corps and volunteered for the Green Berets.

Then, on Feb. 17, 1970, a predawn phone call summoned the Kassabs to Fort Bragg, N.C. "Jeff was in the intensive-care unit of the base hospital," Kassab recalls. "He told us the minute we walked in that Colette and the kids were dead." According to MacDonald, at least four drug-crazed hippies had invaded his home Manson-style, chanting, "Acid is groovy! Kill the pigs!" They had stabbed him and clubbed him unconscious, he said. Then Colette, who was four months pregnant, Kimberly, 5, and Kristen, 2, were viciously beaten and stabbed as they slept.

At first the Kassabs accepted the story. They were indignant when the Army named MacDonald a suspect and initiated hearings into the murders. Testifying as a character witness on Jeff's behalf, Fred Kassab declared firmly, "If I had another daughter, I would want the same son-in-law." By October the Army announced it would not press charges. "After that," says Kassab, "we were lucky ever to reach Jeff on the phone."

Within a month Fred Kassab was questioning his son-in-law's truthfulness. His doubts began, he says, after MacDonald called him to say he had met one of his attackers while out on the town, had beaten him up and then killed him. "He said the police found the body the next day, but since the guy was such a terrible drug addict they didn't pursue the case," Kassab recalls. "I swallowed it all, hook, line and sinker, but my mind was in turmoil." (MacDonald reportedly said later he made the phone call so that his father-in-law would leave him alone.) Puzzled, Kassab managed to obtain a transcript of the Army's closed hearings in the MacDonald case. "When we put the transcript together with the facts as we knew them," says Kassab, "his story began falling apart.

"The funny thing is that he was an amateur boxer in college, but he was held on the living room couch the whole time while Colette fought like a tiger in the bedroom. Jeff says he was stabbed 19 times, but most of his wounds were superficial," says Kassab. To explain the absence of fingerprints on the ice pick and kitchen knives used as murder weapons, Kassab points out, MacDonald told investigators the killers wore rubber gloves. "Why would lunatic hippies think to wear rubber gloves and where did they get them?" asks Kassab. "Why didn't they take the drugs and syringes that were in the hall closet, which was open?"

Kassab prepared a carefully researched brief charging the Army with mishandling the case. "Then I got two of the biggest suitcases you ever saw and personally delivered a copy to every congressman and senator in Washington," he says. Eventually Kassab himself spent seven hours in the murder house trying to re-create the crime. The result, in January of 1975, was MacDonald's indictment by a U.S. grand jury. Kassab, now sales vice-president for an egg company, has lost track of the time and expense he has devoted to his quest for justice. "I'd go into debt for the rest of my life to settle this thing," he vows.

MacDonald, meanwhile, built a new life and a clean reputation. As director of emergency room service at St. Mary's Medical Center in Long Beach, he is widely respected in his community. Friends have rallied to his support, reportedly even mortgaging their homes to help raise his $100,000 bail. Unmoved, Fred Kassab concedes that the trial, expected to run several months, will be a difficult one, but believes "no jury in the world" could find MacDonald innocent. And if this jury can't reach a verdict? "I guarantee," says Fred Kassab evenly, "I'll turn the world upside down making sure he gets retried."