The democrats demanded an investigation. They got one.
Dems thought they were going to have a "gotcha" moment and prove the vindictiveness of the White House. They didn't care whether this was actually a crime. C-Span had a woman on a day or so ago who is a lawyer and a director of some journalists' organization. She said she warned them repeatedly not to pursue this in the fashion that they did and that it was very unlikely that any crime had been committed. But the desire to "get Bush" was so great that they were blinded and hence have been hoist on their own petard, which is really a lovely thing to see.
IIRC, the investigation was started to determine whether or not a crime was committed by the original leaker. However, it apparently was expanded, and some suggest quite broadly, into investigating other leaks. Novak has said that there will be lots of surprises when the story finally comes out, and some of that will be tangential to the original Plame issue.
Lying to a federal investigator is a felony in and of itself - even if the investigation determines that no other crime as been committed, and irrespective of whether someone was under oath when the statements were made - and it's quite possible that one or more individuals at the White House (where Fitzgerald apparently believes the leaks originated) may have attempted to mislead or stall what they saw as unimportant or misguided investigation into a leak they regarded as " business as unusual".
The problem is that once such an investigation gets rolling, and prosecutors start employing such leverage to discover the identities of others involved, those threatened with indictment for misleading investigators are under intense pressure to provide information in trade for leniency, and may start offering up knowledge of other questionable activities by those around them.
And if there individuals, in turn, start to mislead investigators... well... no one knows where things are headed.
Federal prosecutors aren't shy about turning the screws in such cases and Fitzgerald mastered the technique during his tenure investigating political corruption cases in Illinois - cases where apparently minor individual misbehavior (for example taking a bribe to pass an unqualified applicant taking a driving test) eventually lead into the indictment and conviction of dozens individuals and finally to the indictment of the Governor himself.
The point is not that something of this sort is necessarily happening here, but rather that despite the fact that we have not seen any indictments to date, or that to outsiders the case appears to have stalled, the case is possibly still alive, and even growing, And that Fitzgerald's continued request for testimony and other evidence from reporters does not mean that it this point it’s his only, for even his major, means of pursuing the case.