When materials scientist Joseph Michael and his team at Sandia National Laboratories in Albuquerque, N.M., trained their high-powered electron microscope on anthrax spore samples the FBI had sent them in February 2002, they made two crucial discoveries: The first confirmed previous findings that the Bacillus anthracis spores mailed to U.S. Senate offices and various media outlets (shortly after the September 11 terrorist attacks) contained silicon, a substance used to turn anthrax-causing spores into a biological weapon.

But it was Sandia's next discovery that marked a critical turning point in the feds's probe of the mysterious mailings, which killed five people, injured 17 and prompted thousands more who were potentially exposed to the deadly spores to take potent antibiotics—in particular, Ciprofloxacin (known to irritate the gastrointestinal tract and cause joint swelling). Using highly sensitive transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM), the researchers came to a startling realization: The silicon had grown organically inside the Bacillus anthracis samples, nothing had been added to weaponize the spores. "The silicon was not on the outside of the spore," says Michael, who headed up Sandia's investigation, "but rather incorporated on the inside."

It was this key information that helped the FBI to rule out the likelihood that a terrorist organization was behind the anthrax mailings and prompted the agency to turn its attention to U.S. government labs as the possible source of the anthrax. This move eventually led the agency to conclude that Bruce Ivins, a scientist at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID), a federal biodefense research laboratory at Fort Detrick, Md., who initially assisted with the investigation, was the culprit. Ivins, 62, two months ago committed suicide as prosecutors prepared to charge him in connection with the mailings.

Post-9/11 Scare
FBI Director Robert Mueller, III, this week told a House panel that he plans to commission the National Academy of Sciences to review evidence compiled by the agency's Amerithrax Task Force to erase any remaining doubts that the mailed anthrax came from Ivins's lab—and close a case that began seven years ago when a batch of letters containing Bacillus anthracis, the bacterium that causes the disease anthrax, were sent to several news outlets, including the New York Post and NBC News. Although only the letters to the Post and NBC were discovered, the FBI concluded that contaminated letters were also responsible for anthrax infections at ABC News, CBS News and for the October 5, 2001, death of Robert Stevens, a photo editor for the National Enquirer publisher American Media, Inc., based in Boca Raton, Fla.

Anthrax infection begins within a week of exposure with a few days of fever, chills, chest heaviness, malaise and cough as the spores are absorbed by the lungs. Ultimately the bacteria produce toxins that damage the lungs and poison the blood, potentially sending the victim into septic shock that leads to organ failure and, in many cases, death.

By mid-October, spore-filled envelopes had also been discovered in the offices of former Sens. Tom Daschle (D–S.D.) and Patrick Leahy (D–Vt.), along with ominous messages, including: "You can not [sic] stop us. We have this anthrax" and "Are you afraid?" Buildings throughout Washington, D.C., including the Hart Senate Office Building, the main postal distribution facility and several offices of the U.S. Food and Drug Administration were shut down out of fear they had been contaminated. The feds tested 36 postal facilities in the D.C. area (including one where two postal workers died), finding spores at 15 of them. The Hart Building did not reopen until January 2002.

Investigating the Investigators
Congress has called for an investigation of the FBI's work on the anthrax case. One major misstep was revealed Tuesday, when the Los Angeles Times reported that Peter Jahrling, a former senior civilian scientist at the Fort Detrick facility, admitted that he had made an "honest mistake" seven years ago when he told top FBI brass that he believed anthrax spores he examined had been altered to make them more deadly.

Worried that a terrorist organization or a country sympathetic to al Qaeda might be involved, the U.S. Department of Justice in late 2001 commissioned a series of tests to determine whether the spores had been coated with a substance that would prevent them from clumping together, enabling them to hang in the air longer than they would normally, thereby increasing the chance of  inhalation.

Early in the investigation, the FBI appeared to endorse the view that only a sophisticated lab could have produced the material used in the Senate attack, investigative journalist Gary Matsumoto wrote in the November 2003 issue of Science. In fact, in May 2002 16 scientists and physicians working for the government published a paper in JAMA The Journal of the American Medical Association, describing the Senate anthrax powder as "weapons-grade" and exceptional: "high spore concentration, uniform particle size, low electrostatic charge, treated to reduce clumping."

In addition, the August/October 2002 newsletter from the Armed Forces Institute of Pathology (AFIP), a research organization that the government often turns to for help analyzing potentially pathological substances, reported that a mass spectrometry analysis found silica—a staple in professionally engineered germ warfare powders for decades—in the powder sent to Sen. Daschle. The silica was believed to be there to prevent the anthrax spores from aggregating and make it easier for them to disperse into the air, according to Matsumoto, who added that any such weaponized form of anthrax is "more than 500 times more lethal than untreated spores."

Finding the Right Technology
By the time the Sandia researchers began their work in February 2002, "we had heard just like everyone else that the spores had been weaponized," says Michael, who had proposed a study of the elemental composition of any materials found growing outside the spores.

The first step was to find the silicon. Michael was aware that FBI researchers had analyzed the samples with both scanning electron microscopy (SEM), which scans surfaces with a high-energy beam of electrons, and with energy dispersive x-ray spectroscopy (EDS), which analyzes x-rays emitted by a substance after it is hit with charged electrons. But at that point, no one had studied them with a scanning transmission electron microscope (STEM), which transmits a focused beam of electrons through a small part of a specimen to form an image and could provide compositional information by examining the spores a few nanometers (one nanometer is 40 millionths of an inch) at a time, a higher resolution than SEM could provide, Michael says.

This enabled Sandia researchers to not only detect the presence of a foreign substance such as silica, but also to determine its location on or inside the spore. "In the FBI's mind, it was important not only that trace amount of elements were present, but also…[to determine]…where those elements were located in the sample through microanalysis," says Paul Kotula, a Sandia material scientist who studied the samples with Michael.

The researchers could find no way that the silica could be placed inside the spore without leaving a residue on the spore's outermost layer. (They found none.) Instead, the researchers determined that the silica formed inside the spore naturally. After only a month examining the anthrax samples in March 2002, Michael and his team were convinced, contrary to other reports, that the anthrax used in the attacks had not been weaponized.

Some of the samples they worked with came from the USAMRIID,  which employed both Ivins and Steve Hatfill, another government scientist the FBI pursued but who ultimately turned out to be a dead end. (He was vindicated in June when he won a $5.8 million settlement in June against the Justice Department.) According to Michael, neither he nor other Sandia researchers ever worked directly with any of the USAMRIID researchers, instead obtaining all of the samples they tested through the FBI. Nor did Sandia work with live anthrax; all of the samples they received were first inactivated or irradiated by the FBI.

Michael says he was surprised to hear that the feds were closing in on a scientist at USAMRIID (Ivins, who died of a prescription-drug overdose), but that he was "not surprised the person who did this had knowledge of microbiology."

In the end, it was at Sandia where scientists cracked the mystery behind the mailings. The problem was, says Michael, that he had to keep mum on his findings—which might have calmed a jittery nation still reeling from the 9/11 terror attack—until the FBI wrapped up its investigation. "That's been one of the really frustrating things for Paul and me," Michael says. "We knew the answers but couldn't tell anyone"