Early Saturday, and just minutes apart, firebombs destroyed a car outside the campus home of one researcher and torched the front door of another, who had to flee with his wife and two young children by lowering a ladder out a second-story window. A third researcher received a threatening telephone message around the same time, police said.

Federal, state and local investigators are checking for potential links between this weekend’s attacks and other incidents of harassment against UC scientists in recent months. Those include a firebomb that exploded on the front porch of a UCLA researcher’s home in January and six masked activists trying to force their way into a researcher’s Santa Cruz home during a child’s birthday party in February.

It is unclear whether the incidents are the work of a cohesive group or the actions of individuals or small cells inspired by ideology and the online posting of each other’s exploits, said those familiar with the activists.

Animal Liberation Front, a group that took responsibility June 16 for setting fire to a UCLA van as part of its effort to force the university to stop experimenting on primates, has historically operated as small groups that remain anonymous to each other, Vlasak said.