An advisory from U.N. security chief Michael McCann circulated Wednesday urges staff to look out for suspicious letters or parcels "in light of the current crisis."

U.N. headquarters was evacuated after hijacked planes crashed into New York's World Trade Center and the Pentagon in Washington on Sept. 11, and again the following day.

The 39-story New York landmark was reported to be a terrorist target following the 1993 bombing of the World Trade Center. Letter bombs were sent to a journalist based at the United Nations in 1996, and in August, the FBI took away a suspicious package that arrived at the U.N. mailroom.

Since Sept. 11, the area immediately around the United Nations has been blocked off by garbage trucks and security checks have been stepped up. Tourists have been barred from the building but are being allowed to return starting Thursday.

McCann's advisory was issued following the death of a Florida man from anthrax, a bacteria that causes an acute infectious disease and can be used as a biological weapon. Two co-workers have also tested positive, and the case is now the subject of a federal criminal investigation.