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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.

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.”