Tsunao Saitoh had just pulled into the driveway of his California home when the unseen killer attacked. Bullets smashed through the driver's side window of the BMW, riddling Saitoh's upper torso. His 13-year-old daughter Loullie bolted out of the car in a desperate effort to escape; she was shot again and again in the back. When police arrived some hours later, they were confronted with a savage double homicide and no suspects. The Japanese-born Saitoh, 46, was a prominent scientist--an expert on Alzheimer's disease--with no known enemies. Detectives could find no motive for his murder.

To many Japanese, however, there was no mystery: the father and daughter had fallen victim to the U.S.'s violent culture. The incident "again vividly shows the horrors of America, the gun society," editorialized Mainichi Shimbun, a Japanese national newspaper, "and has revived the nightmare of other gun killings of Japanese in the United States." Japanese television networks provided extensive coverage of the Saitoh murders, with reporters filing live reports from the victims' doorstep.

Five days after the murders, police in La Jolla, the affluent San Diego suburb where Saitoh lived in an $800,000 home, remained baffled by the crime. They believe the gunman was lying in wait for Saitoh when he returned home from his laboratory at the University of California at San Diego, where he had been helping Loullie with her homework. "It's a complete mystery," said Lieut. Glenn Breitenstein of the San Diego police homicide unit. "It doesn't appear to be robbery." The family's luxurious home had not been burglarized, he said, and no attempt was made to steal the scientist's car or the cash in his pocket. Saitoh was estranged from his wife Shizue, who was reportedly on a trip to Europe and Japan when her family was killed.

Saitoh had earned a doctorate in mo- lecular biology from the University of Kyoto before leaving for the U.S. He had been a professor of neuroscience at U.C. San Diego for 11 years, and had conducted research previously at Columbia University in New York City. In 1993 Saitoh made headlines in the medical press when he identified a protein in the brain that seems to trigger Alzheimer's disease, the fatal, degenerative brain ailment that afflicts millions of elderly people around the world. He predicted that the discovery would lead to a cure for Alzheimer's within 20 years.

Saitoh was director of U.C. San Diego's Alzheimer's Disease Research Center, where he supervised a dozen other scientists. He received grants for his work from the National Institutes of Health, the Alzheimer's Association in Chicago and from pharmaceutical companies. "He was one of the world's leading researchers in this area," said Phyllis Lessin, deputy director of the research center. "We're all devastated. He was a dedicated, brilliant researcher and a dear, gentle man with a ready smile for everyone."

With the Saitoh murders, 12 Japanese nationals have been slain by guns in the U.S. since 1990. The most shocking incident was in 1992, when a 16-year-old exchange student in Baton Rouge, Louisiana, Yoshihiro Hattori, was shot and killed by a nervous householder who mistook him for an intruder. Costumed for a Halloween party, the teenager intended only to ask for directions. The man who shot him, a meat-cutter named Rodney Peairs, was acquitted of manslaughter after defense attorneys argued that he was legally defending his home.

Japanese noted some disturbing similarities in the Hattori and Saitoh cases. Each victim was in a safe residential area, not indulging in high-risk behavior, yet fell victim to senseless violence. In Japan, where possession of guns by civilians is illegal, murders are extremely rare. In 1995, 32 Japanese were shot to death; in the U.S., which has twice Japan's population, more than 15,000 people were murdered with guns in 1994, the last year for which there are statistics.

The central puzzle for the Japanese and American reporters who converged on San Diego was why a respected scientist like Saitoh would be the target of what resembled a gangland hit. One focus was on the researcher's finances. How, investigators were asking, was Saitoh able to make a $140,000 down payment last year on his house--steps from the La Jolla Country Club in one of the most exclusive communities in California--and take out a $650,000 mortgage, all on a salary of $106,000? He had also reportedly paid cash for his BMW and for a 1992 Mercedes that he owned.

But even in the unlikely event that Saitoh was involved in some nefarious dealings, the brutal manner of his demise jolted the Japanese, whose own gangsters for the most part settle their disputes without deadly violence. "I never even dreamed of the possibility that my son and granddaughter could be killed by a gun," said Saitoh's father Shigezo, an 84-year-old fortune-teller in Urawa City, north of Tokyo. Shigezo Saitoh had not seen his granddaughter in 10 years; she was scheduled to visit Japan this summer. Now his last glimpse of Loullie will be at her funeral.

--Reported by Elaine Lafferty/Los Angeles and Satsuki Oba/Tokyo