Use encryption with a password security set to delete the drive after 5 consecutive invalid login attempts.
The FBI should have separate thumbprint activated usb keys that unlock an encrypted file system on the laptop. eCryptfs would allow for a PKI where the private key is in a tamper-proof usb key. The key could be easily so long that it is uncrackable. If they want more security, they could also include a password. The password wouldn't even need to change more than once a year with the other measures in place.
Then if a thief wants the data, he has to steal the laptop, the usb key, a thumbprint, and a password. Seems pretty secure.