On Mar. 14, a swab taken from an air filter in an Arlington (Va.) post office that sorts some of the Pentagon's mail set off alarms. The reading: anthrax. The office was immediately evacuated and sealed off, as were two other facilities that get mail from the center. Later that day another anthrax alarm went off at a Defense Dept. mailroom in nearby Falls Church, Va. For the postal workers who were dosed with Cipro and sent home that day, the course of events was disturbingly familiar.

In the anthrax attacks of 2001, there were no testing systems in post offices. Until workers actually fell sick, mail continued to flow from post offices to sorting rooms and on to offices, shedding spores along the way. Ultimately, hundreds of people were exposed, nearly two dozen fell sick -- some of whom remain permanently disabled today -- and five died. Cleaning up the 23 tainted facilities took three years and cost $260 million.

This year's incidents proved to be false alarms. Experts believe the lab contracted by the DOD to conduct the tests may have triggered a "false positive" by accidentally tainting the test swab with an anthrax sample already on hand. Yet upsetting as the episode was, there is a consolation: The mail is being watched.

USPS has already installed sniffers in 104 sorting sites and it's on track, by yearend, to outfit all 282 of these centers, completing a $525 million effort. Made by Northrop Grumman Corp. (NOC ), each $150,000 test unit is positioned at a "pinch point" in the mail stream, through which every envelope must pass. There, a vacuum continuously inhales samples into a filter, which is then tested for anthrax DNA. If an alarm sounds, the facility is shut down and the sample is sent for more accurate testing. A positive result would trigger a pre-planned response by local emergency crews. So far -- after 500,000 tests capable of detecting ultratiny traces of anthrax -- none has been found.

With all the machines in place, USPS next plans to scan the mail for other threats. Officials won't reveal which agents, but ricin is a good bet. The poison, derived from castor-bean processing, has been used at least twice to taint government-bound mail. It's no surprise that USPS is in a state of perpetual alert: When the enemy is biological, vigilance is everything.

By Adam Aston in New York

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