The extra key could be set up so that it requires X out of Y keys. Each key could be owned by different organizations, like the state govt, FBI, Courts, Nonprofit oversight committees, citizens oversight of police, and such. This could provide a balance of security and privacy, and allow in extreme circumstances a forced break of encryption.
(Ideally, it would require many orgs that are normally in opposition to agree. It would not be "state govt", "FBI", "CIA" like the old Clipper Chip.)