Strange things happened on Oct. 15 when U.S. Army experts gathered around a microscope in a specially sealed room to examine the anthrax spores that had been mailed to Sen. Thomas A. Daschle: The tiny spores, each one less than one-twentieth the diameter of a human hair, kept leaping off the glass microscope slide as though by magic, then wafting away like weightless wisps of cigarette smoke.

When the scientists tried to weigh the sample, the spores refused to rest on the scale but again became airborne, propelled by imperceptible air movements and tabletop vibrations.

Finally the team dunked some of the spores in liquid chemicals and embedded others in wax just so they could examine and test them. That prevented further losses, but even then investigators ran short of spores long before they had done every test they had hoped to do.

Such are the problems that Army and FBI investigators face as they begin their analysis of a similar letter sent to Sen. Patrick J. Leahy -- an investigation slated to begin today or tomorrow with the meticulously orchestrated opening of the letter and examination of its contents at a military lab at Fort Detrick in Maryland.

The letter -- discovered Nov. 16 in a barrel of unopened congressional mail and leaking anthrax spores "like a sieve," according to an Army scientist -- stands today as the best potential source of clues in the unyielding mystery of this fall's bioterrorist attacks. Experts have spent two weeks devising a plan for opening the envelope so scientists can make the most of its valuable microbial contents and FBI agents can gather fibers, fingerprints or human DNA that may be inside.

In recent days, a team has conducted dry runs on a "body-double" envelope wrapped in tape like the Leahy letter, just to confirm the approach. Several experts said they knew of no previous law enforcement case in which so much planning went into so seemingly simple an act as opening a piece of mail -- an indication of how much is riding on the clues the letter may hold.

"The U.S. Army and the FBI . . . know the sample is precious," said Maj. Gen. John Parker, commanding general of the U.S. Army Medical Research and Materiel Command Center, which is overseeing the analysis. "They want to make every study count toward the end of linking the sample to the perpetrator."

Evidence has been hard to come by in the spate of 18 anthrax cases that started Oct. 1. Three deaths -- one each in Florida, New York and Connecticut -- have not been linked to a known bacterial source. And of the four letters containing spores that authorities have in their possession, only the letter to Leahy (D-Vt.) was obtained unopened and in pristine condition.

The letter to Daschle (D-S.D.) lost some of its contents when it was opened by an aide, and almost all the rest has been used by scientists. A letter received by the New York Post appears to have become damp before being discovered, turning the contents into something resembling "Purina Dog Chow," Army scientists said, and making analysis difficult. And so few spores remained in the letter sent to NBC news anchor Tom Brokaw after its contents were spilled by his assistant that the FBI has asked the Army to delay any analysis, Parker said.

"The Leahy letter is the most intact piece of evidence we have," said FBI spokeswoman Tracy Silberling. "It may be the only complete opportunity we have to study this stuff in detail."

Neither the Army nor the FBI has said precisely what tests are planned for the Leahy specimen, which has been locked in a low-humidity refrigerator at Fort Detrick for the past two weeks. But a detailed look at how the Army analyzed the Daschle letter offers insights.

An FBI agent delivered that letter to Detrick's Special Pathogens Laboratory on Oct. 15, doubly sealed in a pair of plastic bags. A preliminary field test had suggested the presence of the anthrax bacterium, Bacillus anthracis, but visual inspection by experts at Fort Detrick immediately suggested they were dealing with spores that had been processed to a surprisingly fine grade.

In the four years that the special pathogens sample test lab has existed, "this was the first time we had ever received a real impression that this is something to be very concerned about," said Col. Erik Henchal, chief of the diagnostics systems division at the U.S. Army Medical Research Institute of Infectious Diseases, which is home to the lab.

The letter and its surrounding bags were placed inside a third plastic bag and whisked from the biosafety level two (BL-2) facility where the package had first been brought to a more secure BL-3 lab. (The highest level is BL-4.)

That is where the Army's premier anthrax expert, John Ezzell, tried in frustration to look at the powder under a microscope. As spores drifted about, Ezzell began to worry -- about the level of expertise that had apparently been brought to bear in the powder's production, and about the number of spores escaping.

So after transferring a few spores to a microbial culture dish where they could germinate and grow into colonies for genetic analysis, the team put most of the powder away and restricted further inspection to samples immersed in special fluid or embedded in thin slices of paraffin.

A battery of biological assays followed. Tests for antibiotic sensitivity indicated the bugs were not resistant to standard antibiotics. DNA tests confirmed they belonged to the Ames strain, as have all of the terrorism-related specimens. And electron microscope studies of the powder in paraffin showed that the particles were remarkably small -- just 1.5 to 3 microns in diameter -- and consisted almost entirely of purified spores, a perfect recipe for inhalational anthrax.

But there was something else in there, too, and it would require analysis by others to say what. That job fell to a laboratory on the campus of the Armed Forces Institute of Pathology in Northwest Washington. An aging building there is home to a device called an energy dispersive X-ray spectroscope, which can detect the presence of extremely tiny quantities of chemicals.

That device found that silica, but not aluminum, was mixed with the Daschle spores -- an important finding that differentiated the sample from known Iraqi specimens in which spores were combined with bentonite, a mixture of silica and aluminum.

The spectroscope found traces of other elements, too, but there was virtually no specimen left for follow-up studies. One goal of the Leahy letter analysis, Parker said, is to conduct further physical and chemical analyses that may offer clues about the powder's provenance.

Three or four people will probably be in the BL-3 facility when the Leahy letter is opened, Henchal said. Ezzell will be among them, he predicted, and he will probably not wear a protective suit but simply don a surgical mask because he has been vaccinated against anthrax many times. An FBI forensic expert will also attend, while other agents look on through the few windows.

The FBI generally likes to photograph evidence, Henchal added, but that won't be easy. "How do you lay out the material so you can adequately photograph it? It's a real problem with something so lightweight and so fragile."

It is just one of the many ironies of the anthrax murder mystery that something so consequential can be so light.

David Norwood processes a sample that may contain a respiratory hazard. Army and FBI investigators are to begin an analysis this week of a letter sent to Sen. Patrick J. Leahy containing anthrax spores.