In 2001, it was obvious that some kind of early warning system was needed, but none existed. There was some technology in development, but none that worked well enough to monitor (accurately) 24/7 for an Anthrax attack. The detectors are still "on the way." The vulnerability is still there.
Why hasn't such an obvious vulnerability been exploited and an attack made? For one thing, al Qaeda does not have the kind of resources required to produce biological weapons. Some al Qaeda members have been caught trying to, literally, produce toxic substances in someone's kitchen. Moreover, a lot of people are looking for al Qaeda. Even those who are pro-al Qaeda, are not always willing to have an al Qaeda bio-weapons lab in the neighborhood. But the potential is there. It could happen.
The U.S. Postal Service closed eleven Washington DC mail centers on November 6th when an air sample taken at one of the centers indicated the possible presence of Anthrax. It proved to be a false alarm, but reminded everyone that the postal service, or even similar organizations like UPS or FedEx are vulnerable to attack by letters and parcels containing Anthrax. The vulnerability arises from the fact that milled Anthrax spores are smaller than the "pores" in the paper used in most mailing envelopes. Getting hold if Anthrax spores is difficult, but not impossible (it occurs naturally in most parts of the world) and milling them requires fairly low tech equipment. If al Qaeda had some sympathizers working as students or faculty in an agricultural college, the milled Anthrax spores could be produced. The college would also have access to Anthrax vaccine (which many cattlemen and veterinarians use regularly) so that they could work with the spores without killing themselves. The spores would then be transported to the United States (via an airtight container, that could be hidden in, say, a camera), and there transferred to envelopes of various sizes and dropped in mailboxes. If several hundred of these Anthrax letters were mailed at once, from different parts of the country, most of the mail distribution system would be infested with Anthrax before the attack was discovered. In late 2001, five people were killed and 13 hospitalized with Anthrax poisoning when three Anthrax letters were sent to Washington, DC and New York City. Several large mail processing centers were contaminated and had to be shut down for months for clean up. Several hundred letters would shut down the entire postal service for months and kill and injure thousands of people, even if the letters were discovered to contain Anthrax before they were delivered. A similar attack could be made on UPS, FedEx and other parcel delivery systems. This sort of thing would cause billions of dollars in economic losses and great panic.