As classes began this week at many universities, some employees turned on their computers to find bomb threats sitting in their inboxes.

Such threats aren't necessarily unusual at large and prominent institutions, but in this case officials started to notice that they weren't alone. At Princeton University, an e-mailed bomb threat arrived at an inbox for general inquiries early on Sunday morning and was discovered the next day. Eleven separate messages arrived at the admissions office of Oregon State University at around the same time. On Tuesday, American University's Washington College of Law received two threatening e-mails.

In each case, officials swept buildings or evacuated them -- and no bombs were found. Now the Federal Bureau of Investigation is trying to determine whether any of the e-mails were connected in some way, and other institutions are on alert for similar threats. The messages "targeted campuses across the country," said Todd Simmons, director of news and communications services at Oregon State, "like spam e-mails."

Especially at a time when fears over campus security are heightened, in the wake of the shootings at Virginia Tech and the more distant shadow of September 11, universities are quick to take all such threats seriously. So as a matter of course, the FBI investigation could be considered routine whether or not an eventual link between the various threat e-mails is found.

"This is not an uncommon event on campus, obviously elevated due to the real impact of 9/11," said Sheldon E. Steinbach, a lawyer in the postsecondary education practice of the Washington law firm Dow Lohnes. "But they tend to overwhelmingly be hoaxes, and create a copycat syndrome that perpetuates itself for a short while."

At least seven universities had received threats between last Friday and Tuesday, including:

• Carnegie Mellon University;
• Cornell University, where a threat e-mailed to a faculty member at the Johnson Graduate School of Management caused an evacuation;
• the University of Illinois at Chicago;
• Another university in the Southeast (unidentified, but not Duke or Emory) where the admissions office was fully evacuated.

Officials at the Department of Homeland Security and the FBI, where the National Center for the Analysis of Violent Crime's Behavioral Analysis Unit is on the case, would not comment specifically on the investigation. (A DHS spokeswoman said there is "no credible information that would lead us to believe that there is an imminent attack to the homeland.")

But strikingly, there seemed to be some characteristics that at least some of the e-mails had in common:

• Messages didn't mention the institution by name or even single out a specific building, suggesting a lack of familiarity with the campus. At Princeton, said Cass Cliatt, the director of media relations, the threat didn't use the word "Princeton" and only specified the "School of Engineering" -- presumably the School of Engineering and Applied Science, which is not in one building but spread out over several. Officials did walk-throughs of eight buildings.
• The e-mails didn't all arrive at universities that were starting classes this week; Princeton, for example, doesn't begin its semester until later in September.

"After learning that there were other universities [it] actually reinforced in our minds that it was not a credible threat," Cliatt said.

For now, universities will probably continue to cooperate with each other as well as with state and federal authorities. At Oregon State, said Jack Rogers, the director of public safety, the IT department is already working to trace back the origin of the threats.