Several states are filing suit against the US Government over $16 billion worth of Series E bonds that were never cashed in. The states want the funds turned over to the states' custody in order to find the rightful owners. If they can't find the owners, the funds would revert to the state.
By the way, if you have any of the following bonds, they are no longer earning interest and should be cashed in:
Series E - Issued May 1941 through October 1979
Series H - Issued June 1952 through October 1979
Series HH - Issued January 1980 through October 1989
Savings Notes - Issued May 1967 through October 1970
Series A, B, C, D, F, G, J, K - All years