Mr. Mueller is now trying to force the FBI to shed its traditional case-oriented approach to its job. In its place, he hopes to build what amounts to a new agency, a Federal Bureau of Prevention whose central mission is to collect, analyze and act on information that will help prevent attacks.

But it is uncertain whether Mr. Mueller, or anyone, can reorganize an institution whose agents have been trained to solve crimes.

... "Problems have become legendary ... Hopefully it will change, but the agents around the country are demoralized. It could take years to fix."