More than 1,000 biological detectors are sniffing mail across the country for dangerous contamination as the hunt goes on for whoever put anthrax in letters and killed five people just after the Sept. 11 attacks.

An anthrax case in Florida, reported five years ago Wednesday, brought the first hint of what turned out to be contamination of mail that reached Washington, New York, Connecticut and New Jersey and raised fears nationwide.

Last month, FBI Director Robert Mueller said agents are still working on the aging anthrax case, and he declared it "will be solved and the person or persons responsible will be brought to justice."

"From the outset we have been open to any and all theories, and the investigation continues on any and all theories," he said.

The Postal Service has taken action in an effort to prevent a repeat.

"We have fully deployed the fleet of bio-detection systems" on canceling machines at 271 mail processing locations, Postal Vice President Tom Day said in a telephone interview.

A modified version for larger, flat mail items will be put into service next year, he said.

Installation of the current system cost $800 million, provided by Congress, and the post office is spending about $70 million to operate it. That annual cost is expected to climb to $120 million.

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The detectors check for anthrax and two other biological hazards, which Day declined to name.

Among those killed in 2001 were two postal workers at Washington's Brentwood mail processing facility. Day said workers now are trained to look for suspicious packages and call in postal inspectors if they detect something unusual.

Among the things that make a package suspicious are leaking powder and liquids. In addition, there are other telltale signs that the agency does not like to discuss for fear of tipping off terrorists.

Last week, the FBI denied it had overestimated the potency of the anthrax spores used in the killings.

Shortly after the attacks, there were reports that the spores contained additives and had been subjected to sophisticated milling — both techniques used in anthrax-based weapons — to make them more lethal. But bureau officials now say the early media reports of weaponized anthrax were misconceptions.

If the anthrax used was less sophisticated than originally thought, that opens up a wider field of potential suspects.

A small number of people in the U.S. and abroad are being looked at by investigators because they fit some criterion, such as access to anthrax, said one official who declined to be identified because authorities are reluctant to discuss the details of ongoing investigations.

Neither that official nor any others involved with the case would discuss the status of Steven Hatfill, the former Army scientist once described by then-Attorney General John Ashcroft as a "person of interest" in the case. Hatfill has sued the government, alleging that leaked statements about him damaged his career.

Currently there are 17 FBI agents and 10 postal inspectors assigned to the case. Investigators have conducted more than 9,100 interviews, issued more than 6,000 grand jury subpoenas and completed 67 searches.

Most of the time when powder is leaking from a parcel, it turns out to be food, such as flour or baking powder, or perhaps a pill that has gotten crushed, Day said.

When packages leak, it may be because people ship things that should not go through the mail, such as parts of dead animals.

"They deep freeze them and, unfortunately, the dry ice is exhausted and we've had a number of cases where red liquid is oozing from the parcel," Day said.

Despite the installation of the detectors, many postal workers feel not enough has been done, according to William Burrus, president of the American Postal Workers Union.

The units that have been installed are effective, Burrus said. But not all mail is processed in postal facilities, he said. Some is prepared in advance by large business mailers and dropped off for delivery.

So far, the detectors have conducted more than 3 million tests, screening some 60 billion pieces of mail with no false alarms. Postal contractors and the Defense Department worked together to come up with the system.

Asked if any real biohazards have been detected, Day said: "That, you would have heard about."

Replaceable cartridges in the system do the actual tests and run an automatic self-check. If that self-check indicates a problem, the sample is routed to a different cartridge. All the cartridges are connected through a secure broadband network so they can be monitored.