San Jose, Calif. -A killer is stalking the cats of southern San Jose, mutilating 21 felines since May 31 - primarily pets taken from owners' yards at night and returned dead, often dismembered and skinned.

Las week, the killings took a disturbing new trend that is alarming police and investigators, where the mutilator killed two cats in one day, including a kitten that was left in a park for kids to find. the killer is attracting national attention because the acts appear to be directed toward pet owners, who find the murdered animals.

San Jose police have assigned a full-time investigator, Det. Mike Roush, plus backup investigators and are treating this as a felony crime wave.

"I'm not going to give away what tactic or tools we are using, but we are actively and seriously investigating these crimes," said Sgt. Ken Tanaka. He urged residents to telephone 911 if they witness these crimes in progress.

Because of the stalking nature of the killer, many of the owners as well as other residents who own animals have refused to be identified by name. Many say they are fearful for their own safety.

Public focus should be directed at the nature of the criminal behavior, not the fact that the victim is an animal, said Kim Roberts of the Humane Society of the United States in Washington, D.C.

We need to look at the behavior of the perpetrator. This is someone who is torturing, dismembering and killing a living being. That's serious," Roberts said Tuesday.

The power of cruelty in this instance is directed not only at the cat but also at it human owner.

Human Society of the U.S. research show that in almost 40 percent of such cases, the killers progress from animals to people.

"If I were in the community I would be worried, worried not only for other animals there but for children," said Dr. Frank Ascione, professor of psychology at Utah State University and author of two books on how cruelty to animals frequently escalates to violence against people.

Arthur Gary Bishop, executed in Utah in 1988 for the torture and s exual mutilation killings of five boys, often practiced by killing puppies he adopted, Ascione said.

Most serial killers, including the Boston Strangler, were also animal abusers, he said. "Evan people who don't care about animals should care about cruelty-to-animal cases because of the progression from animal to humans," said Christine Franco, the case investigator for the Humane Society of Santa Clara Valley.

A $15,000 cash reward for information is being offered by the society.

So far no pattern other than the general neighborhood has emerged. Among the things already examined are past cases, the gender and ethnic heritage of the animals' owners, Tanaka said.