EMERYVILLE, Calif. -- Two small bombs shattered windows early Thursday morning on the campus of biotechnology company Chiron Corp., authorities said, but nobody was hurt.

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Noreen Jaramillo With The Latest From The Chiron Plant

Nobody claimed responsibility for the blasts, which caused minimal damage, police said. Chiron's 2,000-employee campus was closed until about noon.

While police and company officials declined to speculate on who may have been behind the bombings, Emeryville-based Chiron and its executives have been confronted in recent weeks by animal rights activists protesting the company's relationship with Huntingdon Life Sciences Inc., which tests drugs on animals.

Activists have noisily protested in front of executive's San Francisco Bay area homes in the early morning, harassed their neighbors and vandalized at least one Chiron executive's car, authorities said.

Chiron has increased security since the periodic protests began in May. On Wednesday, the company circulated an e-mail warning workers of the harassment.

Chiron makes drugs, vaccines and blood tests and is required by the Food and Drug Administration to test many of its products on animals before it can sell them, Chiron spokesman John Gallagher said.

Emeryville Sgt. LaJuan Collier said the first bomb exploded shortly before 3 a.m. in front of one building on the Chiron campus and the second detonated about an hour later in front of another building. The buildings appeared to be unoccupied at the time, Collier said.

Damage to both buildings was minor, Gallagher said. Gallagher declined to discuss what type of work is conducted in the damaged buildings and would not comment on whether Chiron conducts animal research at the Emeryville campus.

The FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting police with the investigation.

Chiron, which employs 4,000 people worldwide, is one of only few profitable biotechnology companies. It has four drugs approved for sale in the United States that treat cancer, cystic fibrosis and multiple sclerosis.

Chiron contracts with Huntingdon to test drugs that are still under development.

Huntingdon performs tests on about 75,000 animals a year, mostly on rodents but also dogs and primates, at its labs in England and in East Millstone, N.J.

Huntingdon has been the target of a four-year campaign waged by animal rights activists who allege the company mistreats its research subjects. Huntingdon officials at the company's East Millstone lab didn't return a telephone call seeking comment.

"We are committed to providing the highest levels of animal husbandry and welfare," the company's Web site states.

Most of the animal rights activists who have been harassing Chiron employees are affiliated with a group called Stop Huntingdon Animal Cruelty, commonly referred to by its acronym SHAC.

SHAC spokesman Kevin Jonas said he's unsure if Thursday's bombing was related to any animal rights protest.

"But it looks like an action that we would support," Jonas said.