Let me register a slight disagreement with Byron. Under federal law, a person may be convicted on the uncorroborated word of a single witness — even if the witness is an accomplice. This happens more frequently than you would think. That’s because, at least in multi-count indictments, many individual counts (or criminal transactions) are joined together under an overarching count in which the individual counts are subsets. In the Libby case, the overarching count was obstruction of justice, and it was like a generalized scheme or framework that incorporated the four remaining charges about lying to the FBI agents and to the grand jury on various occasions.

The reason this often happens is not hard to see. Let’s take a drug case. If the whole case is about a single transaction — say, one cocaine deal that happens in a fleeting instant on a street corner. If that’s all there is, and the government has only witness who said he saw it or participated in it, but it has no other proof (e.g., the drugs, photos, bank records, etc.), that is a flimsy count indeed — although the jury is free to convict if they believe the witness beyond a reasonable doubt. But if there are five counts, one saying the defendant was involved in a drug conspiracy of some duration and four other ones charging individual drug sales within the conspiracy, then it’s not so flimsy anymore. As the saying goes, pieces of evidence are not looked at in isolation; the case is considered as a whole. If you believe there was a drug conspiracy, it becomes a lot more likely that the guy did the individual drug deals, even if the evidence supporting any single one of them is not overwhelming.

Finally, false statements to an agent are harder to prove than grand jury perjury, for reasons Byron has alluded to. The FBI’s procedure is to take notes of interviews, not record them. In the grand jury, there is always at least a stenographer, and sometimes a videotape. Juries will often give the benefit of the doubt to a defendant on false statements, especially if the agents notes or independent recollection are not thorough. It looks like that’s what happened here on the acquitted count. By contrast, in the grand jury, there is not uncertainty about what words were said (although there may be lots of uncertainty about meaning and intent).