At least 13 U.S. universities, including Princeton, MIT and Carnegie Mellon, have been targeted for anonymous e-mail bomb threats as students return to classes, federal and local law enforcement authorities tell the Blotter on ABCNews.com.

One of the schools, Clemson University, in Clemson, S.C., was targeted again today and an evacuation was ordered, ABC News has learned.

Photos: Back-to-School Bomb Threats

"We have had three in the last 10 days," Clemson University spokeswoman Robin Denny said of the bomb threats. She said the university has been sending out e-mail alerts to students, faculty and other personnel, saying evacuations were in progress after each threat.

The FBI, regional authorities and university police are investigating the spate of threats that include a set of difficult-to-track threats that use Internet remailer services to eliminate the sender address and render the threat anonymous and more difficult to trace, the FBI confirmed.

"The FBI is aware of the spate of bomb threats," FBI spokesman Richard Kolko told ABCNews.com. "Working with university police and our field offices, we are investigating."

The threats, timed to the resumption of classes at schools around the nation, may simply be the work of pranksters, but officials are taking no chances.

Click Here for Full Blotter Coverage.

Last week, New York state's Office of Homeland Security issued an alert to law enforcement and university officials based on a review of threats in the education sector, Michael Balboni, New York's deputy secretary of state for homeland security told ABC News.

"We have been issuing intelligence alerts for some time; we do it sector by sector," Balboni said. "After the Virginia Tech shootings, campus violence is on everyone's mind...we thought the beginning of the school year was a good time to look at the issue."

According to Balboni's alert, "It appears that some of the messages were sent using anonymous remailer services. A remailer is an address through which electronic messages pass without exposing the sender's origin or identity. The message is first sent to the remailer before reaching its actual destination. The user ID and other information pertaining to its origin is removed, replaced by an anonymous ID and the message is sent."

"The source of the e-mails, including the remailers, will be investigated by the FBI," Kolko said.

According to federal law enforcement, Clemson University, Middle Tennessee State, Western Illinois University, University of New Hampshire, the University of Iowa, Princeton University in New Jersey, Carnegie Mellon University in Pennsylvania, University of Alaska at Anchorage, the College of William and Mary in Virginia, MIT, Cornell University in New York and Oregon State University have all received threats.

Following the Aug. 27 threat, MIT police "immediately notified the MIT Operations Center and Safety Office, the Cambridge police and fire departments and other educational institutions, and continue to investigate the threats," the university said in a statement issued at the time and resent to ABC News today. "The MIT police see no immediate cause for concern and point out that unspecified threats of this nature are not uncommon and that many other universities received similar threats today."

Several of the threats indicated that explosives or pipe bombs were placed either inside or close to a campus building, according to the New York bulletin, federal authorities and university officials.

Since the bulletin was issued, according to New Jersey law enforcement sources, Princeton has received yet another threat.

"The Department of Public Safety is investigating two e-mailed bomb threats received by the University in the past week and deemed to be hoaxes," a campus announcement said on the Princeton Web site. "The first threat received was sent to a single e-mail address, but the second was sent to multiple general e-mail boxes serving the University."

Authorities in New York said that at Cornell University in Ithaca, N.Y., a faculty member reported receiving an Aug. 28 e-mail bomb threat in Sage Hall, which houses the Johnson Graduate School of Management. No devices were found. The Cornell University police department continues to investigate the incident.

In New Jersey, authorities noted that schools, including Rider University, have received threats this summer. In each case, authorities said, searches of the institutions found no evidence of explosives.

At Rider University, a July 20 threat prompted an "Alert for Enhanced Security Measures" address to the "campus community" by university president Mordechai Rozanski.

"Rider University recently discovered a bomb threat against the Lawrenceville campus for Friday, July 20. As soon as the University became aware of the threat, our Department of Public Safety contacted Lawrence Township Police to investigate...Since then, we have been working closely with township and state authorities to enhance security measures and heighten vigilance," Rozanski said.

In Virginia, authorities said that four suspicious e-mails were sent to the College of William and Mary in Williamsburg on Aug. 28. The College of William and Mary police department said the four e-mails were sent to a faculty member at the school of business. The messages, sent with the subject "ALERT," stated there was a booby-trapped pipe bomb in that building and that it would "explode as soon as it was touched." Officers responded to the two buildings that house the business school and found nothing.

In Washington, D.C., on Aug. 29, American University police were informed of an e-mail threat. That case is still open.

At the University of Akron in Ohio, on Aug. 29 and 30, 2007, two separate e-mail threats were received. One came in to the College of Engineering; the other was received at a campus radio station. An arrest has been made in the campus radio incident; the two cases do not appear related. 

Akron University officials notified campus community members on Tuesday that it had "received more e-mail bomb threats over the weekend, all from the same source and within a 30-minute period late Friday and early Saturday," according to an announcement on the university's Web site. The Web announcement went on to say the campus police had conducted "a sweep of the public areas" of two university buildings and "have given the all-clear for both buildings."

Balboni said that the spate of threats appeared to be "outside the norm."

"Historically, we get these bomb threats at the end of the school year," he said.

This post has been updated.

Do you have a tip for Brian Ross and the Investigative Team?