In the hills above the University of California's Berkeley campus, nine protesters gathered in front of the home of a toxicology professor, their faces covered with scarves and hoods despite the warm spring weather.

One scrawled "killer" in chalk on the scientist's doorstep, while another hurled insults through a bullhorn and announced, "Your neighbor kills animals!" Someone shattered a window.

Borrowing the kind of tactics used by anti-abortion demonstrators, animal rights activists are increasingly taking their rage straight to scientists' front doors.

Over the past couple of years, more and more researchers who experiment on animals have been harassed and terrorized in their own homes, with weapons that include firebombs, flooding and acid.

Scientists say the vandalism and intimidation threaten not just themselves and their families but the future of medical research. Specialists in such fields as addiction, eyesight and the aging brain have been targeted.

"It used to be everyone was worried about their laboratories being broken into and their data being destroyed, their animals being taken away," said Jeffrey Kordower, head of the Society for Neuroscience's animal research committee. "What they've decided to do now is make things more personal."

Accompanying the attacks is increasingly tough talk from activists such as Dr. Jerry Vlasak, a spokesman for the Animal Liberation Front press office. In an interview with The Associated Press, he said he is not encouraging anyone to commit murder, but "if you had to hurt somebody or intimidate them or kill them, it would be morally justifiable."

The Washington-based Foundation for Biomedical Research said researchers were harassed or otherwise victimized more than 70 times in 2003, up from just 10 the year before. The number of attacks has held steady or risen ever since, according to the group.

Activists say the escalation in tactics results from a frustration that nonviolent methods have failed to stop what they call the needless torture and killing of animals.

"An animal has as much of a right to life as we do. To take a life without provocation is immoral, it's violent, there's no excuse for it," said Jacob Black, 23, an organizer of demonstrations at the homes of UC Berkeley researchers. "To name and shame these people as morally bankrupt individuals in our society is key."

A Web site aimed at Berkeley lists the names of a dozen researchers and their home, work and e-mail addresses, their photos, and often their home numbers. The roster also includes graphic descriptions of each scientist's purported work with animals.

"This information is here so that others may pressure these individuals with legal protests — we do not participate in or encourage illegal activity," the Web site says.

Despite that disclaimer, the late May protest in the Berkeley hills left a window of the toxicology professor's home shattered along with the window of a neighbor, who sprayed demonstrators with a garden hose to drive them away.

Many scientists are reluctant to discuss the effect these incidents have had on biomedical research. They worry that any sign the attacks are succeeding could just lead to more of the same.

But at least one researcher decided the pressure was too much.

In 2006, activists began besieging the homes of several UCLA professors. Masked protesters converged on scientists' homes late at night, banging on doors, throwing firecrackers and chanting, "We know where you sleep," according to court documents.

Threatening calls and e-mails followed. Firebombs were left near homes three times; two failed to go off, while the third charred a front door. One professor's home was flooded when a garden hose was shoved through a broken window.

During the onslaught, which lasted two years, a UCLA scientist with small children informed protesters he had stopped doing animal research.

"Effective immediately, I am no longer doing animal research," vision researcher Dario Ringach wrote in an e-mail. "Please don't bother my family anymore."

Though no one has been seriously hurt since the jump in home protests, the attacks have drawn the attention of the FBI. The agency has broad authority to investigate animal rights incidents under the Animal Enterprise Terrorism Act of 2006.

"We consider this to be a serious problem, especially when people's lives are being disrupted," said agent David Strange, who oversees a domestic counterterrorism squad at the FBI's Oakland office. "We call it terrorism because it is a violent act violating federal criminal laws that has a political or social motivation to it."

Six members of a Philadelphia-based organization were sentenced to federal prison after they and the group itself were convicted in 2006 of using a Web site to incite threats, harassment and vandalism against people connected with a company that tests drugs and household products on animals.

But otherwise, few activists have been prosecuted, because of free speech concerns and the movement's extreme secrecy.

Recently, federal investigators joined a probe into an alleged February assault against the husband of a University of California, Santa Cruz breast-cancer researcher who experiments on mice. Police said masked activists pounded on the family's front door during a birthday party for their young daughter, and one threw a punch when the husband tried to force them to leave.

Afterward, UC Santa Cruz Chancellor George Blumenthal backed a proposed state law that would limit activists' access to public information about animal experiments. Blumenthal called acts against animal researchers "the greatest threat to academic freedom that I've seen in the history of this campus."

Activists say researchers drill holes into the skulls of monkeys and cats in pursuit of esoteric discoveries that will never help anyone.

But scientists say every effort is made to minimize the suffering of animals used in experiments. Rigorous government and university regulations provide detailed protocols for the humane treatment of lab animals. And scientists must show they have exhausted all other options to obtain data before they turn to animals as test subjects.

In Kordower's work, drugs are used to induce symptoms of Parkinson's in monkeys, which are then given experimental treatments. Afterward, the monkeys are anesthetized and killed and their brains dissected. The research, says Kordower, director of neurobiology at Rush University in Chicago, has led to clinical trials for promising genetic therapies to treat Parkinson's.

Kordower has not faced attacks or protests. University of Utah neuroscientist Audie Leventhal has.

The Animal Liberation Front claimed responsibility for dousing a home owned by Leventhal with glass-eating acid and covering it with animal rights slogans. Leventhal estimated the damage at $20,000. At another home, the group claimed responsibility for putting glue in the house's locks and pouring salt to destroy the front lawn.

Leventhal said he will never abandon his research into the effects of aging on the brain. In Leventhal's experiments, anesthetized monkeys are paralyzed, put on life support and shown flashing patterns on a screen as implanted sensors measure brain activity.

"Even if I retire, I'm going to tell them I didn't retire," said the 56-year-old scientist. "There's no way they're winning."

Still, he said, he has mostly been living out of state since the protests began two years ago. He said he refuses to teach classes to avoid having a fixed time and place where activists can find him. His wife got so scared after activists scaled the gate at their home in a Salt Lake City suburb that she bought a gun, Leventhal said.

"I can see what they've done to me — if it gets enough publicity — preventing people with half a brain in graduate school from doing what I do," he said.