What to do with bonds with one listed person deceased and the other whereabouts unknown

My grandfather has been holding onto a bunch of bonds his now deceased wife purchased for their grandson. We haven’t seen or heard from him in about 20 years. Grandson is listed on all of the bonds and either his deceased father or deceased grandmother is the second person listed on them, so we can’t do anything with them as far as we can tell. Please let me know if we are mistaken.

If we can’t do anything with them, we don’t want to have to hold onto them in perpetuity just for risk of damage or loss, so what do we do with them? Can we just mail them to the Treasury Department for them to hold onto or disperse? We literally have no idea if he’s living or dead, in prison or not, no leads on his whereabouts, etc.

Thanks in advance.