SANTA CRUZ -- The FBI has been called in to help with the investigation into a weekend home invasion at a UC Santa Cruz biomedical researcher's house, while authorities said Tuesday that the researcher and several others have been targeted by animal rights activists this month.

No arrests have been made, no suspects have been named and police have not linked the masked intruders that went to the researcher's Westside home Sunday to the previous incidents, which included vandalism.

"We're going to look at other vandalisms that might have occurred over the last couple of months to determine if there's a connection," Santa Cruz police Lt. Rudy Escalante said.

Three incidents, including Sunday's, have been reported in the city in recent weeks, and at least one in the unincorporated part of the county, according to UCSC spokesman Jim Burns. The vandalism involved chalk writings with animal rights messages on or near the homes of two faculty members, one staff member and one graduate student, he said.

Sheriff's Sgt. Fred Plageman said deputies have taken two or three reports of threatening vandalism at homes of UCSC researchers in the unincorporated county. He said there was a "commonality" between the threats made in those cases but was not specific about what had been written.

"Obviously, these people are capitalizing on fear and intimidation," Plageman said.

However, Escalante said none of the vandalism indicated Sunday's violence was in the works.

No group has claimed responsibility for any of the incidents, authorities said. Santa Cruz police contacted the FBI on Tuesday to ask for help, Escalante said.

Detectives have been investigating possible links between the Sunday attack -- six masked people banging on the front of a UCSC researcher's house and hitting the man who opened the door -- and previous incidents involving animal rights activists in Southern California.

During the prior incidents in the county, at least once person with a bullhorn stood outside a home and shouted at the occupants, Burns said. Also, in the unincorporated area, the assailants opened a gate and trespassed to scrawl intimidating language on a driveway, and in January intimidating calls were left on a researcher's home answering machine.

"Beginning with the chalking incidents ... when we learned about them we encouraged the people to contact the jurisdiction in which they occurred and in every case they did that," Burns said.

UCSC Police Chief Mickey Aluffi spoke about the incidents at a monthly meeting of the county police chiefs, Burns said.

Sunday's incident was the only known physical clash involving activists and UCSC staff.

Those involved in the incident could face charges of stalking, attempted burglary and conspiracy, authorities said.

District Attorney Bob Lee, whose office assisted in obtaining the search warrant, said he thought the nature of the home attack -- there was a child's birthday party going on in the house when it occurred -- was particularly egregious.

"That's no discussion. That's not freedom of speech. That's a criminal act," Lee said. "We will file those charges."

Police have not given a time line for making arrests. Escalante said detectives Tuesday were combing through cell phone records, computer files and other evidence seized from a Riverside Avenue house. Police said evidence recovered in the home pointed to other "possible attacks."

Police raided the home Sunday night after tracing the intruders' getaway car back to the house. The vehicle was found in downtown Santa Cruz on Sunday night and impounded.

He said detectives don't have enough probable cause to arrest anyone yet.

"We don't want to go out and arrest the wrong people," Escalante said.

There were five people inside the Riverside Avenue house Sunday night when officers served the search warrant. Escalante declined to say if any of the two men and three women in the home were suspects and would not release their names. Police plan to release sketches of the suspects this week.

No animals rights group has taken responsibility for the weekend attack, though photos of the Riverside Avenue police raid have been posted on an activist Web site associated with the Animal Liberation Front and the UCSC tree-sit.

Andrea Lindsay, a spokeswoman for the San Francisco-based SHAC 7 animal rights group, said her members -- six are in prison for activist activities -- had no involvement in Sunday's incident. She said well-known animal rights activist Peter Young gave a talk Saturday night at the Louden Nelson Community Center to raise funds for the legal appeals of SHAC 7 members, but she said she did not know of any connection between the event and the home invasion.

Lindsay said, "I understand why people are outraged at the use of animals in experiments and why people use a variety of tactics." She said most animal rights activists abide by "a strict code of not injuring people" during their demonstrations.

Young, a Los Gatos native who served two years in federal prison for releasing thousands of minks from Midwest farms, told the Sentinel on Tuesday he had no involvement in Sunday's incident. He said no one at the SHAC 7 fundraiser Saturday indicated they were planning any activity against a UCSC faculty member.

But Young, who said he moved to Santa Cruz a month ago after getting off probation, said he was present when police raided the Riverside Avenue home of the three UCSC students. He said he went to the house after hearing from friends Sunday that police were targeting the home of fellow activists, but said he didn't know about the home-invasion investigation.

While he said he does not know exactly what happened Sunday, he said, "After being around animal rights activists for many years, I've never encountered anyone who condoned violence against people for the cause of animal liberation."

He said the case seemed to be a "smear campaign by vivisectors to discredit the work of activists. I'm quite sure somebody didn't try to break into the house. We're against violence."

Contact Jennifer Squires at 429-2449 or jsquires@santacruzsentinel.com.