Attorneys on both sides of the FBI's anthrax case against microbiologist Bruce Ivins acknowledge much of the evidence is circumstantial. Paul Kemp, Ivins' attorney, has said that would not have been enough to convict his client in court. The government argues otherwise.
Since Ivins committed suicide late last month, there may never be a clear answer. However, the fact that a case depends on circumstantial evidence does not mean that a crime cannot be proved.
"Circumstantial evidence is just evidence of an indirect nature," says Mary Cheh, a law professor at George Washington University. It allows you to infer conclusions from the available facts, she says.
For example, imagine that a man and a woman check into a hotel and emerge the next morning, night after night. "Maybe I don't have direct evidence that they're having an affair," Cheh says, "but I infer rather strongly from those facts."
In fact, the couple could be meeting to plan a church picnic. In an adultery trial, a defense lawyer might make that argument. The jury would have to decide whether the argument is plausible.
The law says even murder can be proved without a body. In that case, Cheh says, "you have to infer that there's been a death and that it's been murder."
Tom DiBiagio, Baltimore's former U.S. attorney, says that "probably 90 percent of the cases that are tried every day" depend at least in part on circumstantial evidence.
Some circumstantial evidence can be incredibly damning, while other evidence can be less so. The system relies on jurors to evaluate whether the entire body of evidence proves guilt beyond a reasonable doubt.
That's one reason many people are dissatisfied by the conclusion of the anthrax mystery: Ivins committed suicide and the government has moved to close the case, so a jury will never decide whether the government had enough evidence for a conviction.
"The jury here is the public at large," says Donald Stern, a former U.S. attorney in Boston. The public "can draw its own conclusions based upon the evidence the government has presented and the arguments and contrary positions that his defense lawyer has taken," Stern says.
The Ivins case is even more ambiguous because the FBI had not completely finished its investigation when he committed suicide.
On Thursday, a judge approved search warrants for two public library computers that FBI agents watched Ivins use days before he killed himself.
An agent said the computers could contain suicide letters, photographs or other information relevant to the investigation.
When Jeff Taylor, U.S. attorney for Washington, D.C., described the governments evidence against Ivins at a press conference this week, he made it sound as though the FBI had all that it needed.
"Based upon the totality of the evidence we had gathered against him," Taylor said, "we are confident that Dr. Ivins was the only person responsible for these attacks."
While that may be so, the government wanted to be even more confident. Officials were talking with other people who knew Ivins, and they had arranged a meeting with his lawyer in which they were going to urge Ivins to confess.
Had Ivins done that, the circumstantial evidence would have been irrelevant. Instead, he maintained his innocence and overdosed on painkillers, prematurely ending the anthrax investigation seven years after it began.