Investigating Leaks

In "Investigating Leaks" the New York Times writes:

Congress specifically barred prosecuting a journalist who got the name of a covert operative from a government official. Consistent with that, the Bush administration should not use the serious purpose of this inquiry to turn it into an investigation of Mr. Novak or any other journalist, or to attempt to compel any journalists to reveal their sources.

What if the journalist was lying, as in the case of the New York Times "journalist" Jayson Blair? What if the journalist writes that it came from the White House, but in fact it came elsewhere? How can you take an investigation seriously if it turns a blind eye to half of the story?