The AP reports on who is conducting the CIA-Plame investigation:

For now, the investigation is being done by FBI agents in the counterintelligence division, based at the FBI Washington field office, and overseen by 11 career prosecutors in the counterespionage section of the Justice Department's criminal division.

The article says Ashcroft could decide to go with a special prosecutor:

While the administration appeared cool toward naming a special counsel, Ashcroft has not ruled out that possibility, a senior law enforcement official said.
That decision will depend on a number of factors, such as whether a suspect is identified who presents a potential conflict for the Justice Department

News execs and reporters have concerns about subpoenas:

News executives expressed concern that the investigation could lead to subpoenas of reporters' notes and phone records — and the journalists themselves. "The question really comes down to whether there are other ways to do this that do less damage to the idea of the First Amendment, said Bill Felber, editor of The Manhattan (Kan.) Mercury, who handles freedom of information issues for the Associated Press Managing Editors. "This ought to be last resort, not a first resort."