An attack on a Harvard Medical School security guard occurred yesterday morning, only days after an animal rights group sent threatening letters to eight Medical School faculty members.

The guard was patrolling the area behind the New England Regional Primate Research Center--located on the Medical School's Southborough campus--at 1 a.m. when he noticed "motion" in a wooded area, according to a Medical School news release. When he left his car to investigate, someone threw several rocks at his car.

The guard was not injured but the rocks broke the passenger-side front window of his Ford Escort and dented the hood, according to the release.

The incident attracted the attention of local and state police when it was inaccurately reported over the police scanner that shots had been fired, said Harvard spokesperson Joe Wrinn.

The scanner report brought in police from Marlborough, Southborough and Framingham in addition to a State Police K-9 unit, according to the release. Wrinn said the initial report of gunfire was "quickly corrected."

Eight faculty members, including five at the Southborough animal research facility, received letters containing razor blades between Monday and Wednesday, believed to be sent by a radical animal rights group called "The Justice Department."

Despite the fact that the attack occurred near the research center where five of these faculty members work, officials said they found no link between the incident and the letters.

"There is there is no evidence whatsoever" that the attack was related to the razor-blade mailings, Wrinn said.

But heightened concerns, combined with the faulty scanner message, prompted a thorough investigation, Wrinn said.

"Given the time and proximity of each [incident], it would be natural to investigate something like this," Wrinn said.

In response to the incident, Harvard Police stationed two extra officers on the Southborough campus. City and state police stayed on the scene until 3:30 A.M.

The 20-year-old security guard, who asked not to be named, works for Pinkertons Inc., a private security firm.