Not Good Enough for Government Work?

A report in today's News York Times on the federal government's plans for a new law enforcement computer network notes in passing that

for the first time, the Federal Bureau of Investigation and other agencies linked by it will be able to send one another encrypted e-mail. Previously, security concerns about the open Internet often caused sensitive information to be faxed, mailed or sent by courier.

Maybe I don't fully understand the FBI's security needs, but what about commercially available encryption software? If PGP, for example, is so hard to break that sending it outside the country could be considered a "munitions" export, why isn't it secure enough for the FBI?