One enduring mystery of the anthrax attacks is that there were actually two sets of attacks -- with two very different qualities of anthrax.

A first set of letters was sent to NBC and other media outlets in September 2001. The anthrax in those letters was “clumpy” and like “Purina Dog Chow.” Three weeks later, another batch of anthrax-laced letters went out to Senators Tom Daschle and Patrick Leahy. The anthrax in the second round of attacks was much more refined. (Contrary to early, official speculation, the second batch doesn’t appear to have been “weaponized” but it was powder form, making it much more dangerous.)

If there indeed was a single attacker, how did he end up with two such different batches of anthrax?

The Washington Post has a potential explanation. Citing “sources briefed on the case,” the Post says:

Bruce E. Ivins, the government's leading suspect in the 2001 anthrax killings, borrowed from a bioweapons lab that fall freeze-drying equipment that allows scientists to quickly convert wet germ cultures into dry spores.”

The device is known as a “lyopholizer.” And while the Post cautions that Ivins did do “at least one project” for the Pentagon that “might have given him reason” for checking out a lyopholizer, it also says Ivins' department generally stayed away from the devices:

Scientists and biodefense experts familiar with USAMRIID's procedures say that Ivins's department rarely used such freeze-dryers, because the researchers there worked with anthrax bacteria in a liquid form.

"Dry anthrax is much harder to work with," said one scientist familiar with Ivins's lab. A lyopholizer would fit inside the ventilated "biosafety cabinet" at the lab and could have been used without drawing notice, the scientist said. The machine could have processed a few small batches of anthrax liquid in less than a day, he said.

As it happens, we came across a study Ivins co-authored in 1995 about an anthrax vaccine that used “lyophilization,” meaning that’s another reason he might have used the device. But the Post also suggests that he might not have even needed a lyopholizer to bake up the two different batches of anthrax. Instead, according to one expert, he could have used a kitchen oven. And as most bakers know, it often takes a few tries before you get it right:

Other biodefense experts noted that the drying step could have been carried out with equipment no more complicated than a kitchen oven. "It is the simplest . . . but it is the least reproducible," said Sergei Popov, a former Soviet bioweapons scientist who now specializes in biodefense at George Mason University. "If you go too fast you get 'sand,' " he said, referring to the coarser anthrax powder used in the first attacks, in September 2001.

The second batch of letters contained a much finer powder. "To me, it all indicates that the person experimented with the ways to dry the spores and produced small batches -- some of them not so successfully -- he later used to fill up different envelopes," Popov said. "The spores are naturally clumpy. As I understand, he just overbaked the first batches."

This morning’s New York Times also has intriguing details on the investigation. But whether the paper is skeptical of the significance or simply doesn’t have additional details, it keeps its reference to the lyophilizer to a single sentence.

The Times spends more time on another thing that might have helped crack the case. It’s all about the beef broth:

Analyzing traces of the beef broth used to grow the anthrax, scientists measured carbon-14 left from nuclear weapons tests in the 1950s, whose quantity diminishes every year.

By calculating the ratio of carbon-14 to the normal kind in residue of plants eaten by the cow from which the broth was made, investigators learned by June 2002 that the anthrax had been grown within the last two years.

All of which adds up to…who knows. As the Times emphasized yesterday, most of the case appears to be circumstantial. And perhaps more importantly nearly all the tidbits that have been aired have come from anonymice. And that includes reports that the FBI might present its case against Ivins as soon as tomorrow.