Four identical envelopes containing white powder and letters with the words "Black Death" were found in the last two days, two in the engineering building on Monday and two on Tuesday. The return address on all four envelopes is in Idaho.
The substance inside the letters has not been identified. It was tested and determined non-hazardous, but was sent to a crime laboratory for further testing.
Hazardous-materials teams responding to the campus prepared for the worst, setting up a decontamination zone and sealing off the area.
"They turned off the heating and air-conditioning units and they closed down the elevators so that nothing is spread," said Cathy Lawhon, a UCI employee.
University spokesman Tom Vasich said the assistant dean of biological sciences and a computer sciences counselor opened the letters addressed to them Tuesday, prompting the evacuation of two offices in the natural sciences and computer sciences buildings.
Two other buildings were evacuated Monday when a professor in the social sciences department and a professor in the social sciences department opened similar letters.
The threatening letters were addressed to the UC Irvine staff and are now in the hands of the FBI.
No one was harmed by the incident, but authorities are analyzing the powder. Authorities said initial tests of the substance were negative for biological hazards.
"They're in standard legal size envelopes and, basically, the message in it just refers to black death. There's nothing personal to the professors or to the recipient," said Lawhon.
School officials don't know why the professors were targeted. The only thing the four targeted employees have in common is that they are women.
The campus is now scanning all mail more carefully.
"Warn them to look for and better screen the mail and look for these markings and so forth, but the campus receives a lot of mail and it's very simple to have an envelope that's slightly smaller than another one gets stuck in between, and somehow this made it through," said Orange County Fire Battalion Chief Michael Boyle.
Students Tuesday were angry to see more disruptions on campus as the new semester began.
"It's kind of nerve-racking because you don't know if it's going to be in your class or anything. You're kind of nervous of seeing anything like that because it's scary," said student Freddy Lopez.
Officials cautioned students and faculty in an e-mail to keep an eye out for suspicious mail, particularly if:
- It is unexpected or from someone you do not know
- Is addressed to someone no longer with the university or otherwise outdated
- Contains inaccuracies in the recipient's name or title
- Has no return address or one in which the city/state does not match the postmark
- Is of unusual size, weight or is oddly shaped
- Is marked with restrictive endorsements such as "personal" or "confidential"
- Has strange odors or stains
- Is wrapped unprofessionally or looks markedly different than mail typically arriving in your office
Authorities are looking into the possibility that the letters are tied to some type of publicity stunt. There is a movie coming out in February titled "Black Death." There is no direct link at this time, but authorities are looking into it.
The Associated Press contributed to this report.