The Social Security Adminstration will forward a letter to a relative in an emergency.
Here is some info from the Social Security Administraion's site:
The Social Security Administration cannot provide you with an address of someone without his or her permission. However, SSA has a letter forwarding service that can be used to attempt to contact a missing person. We can attempt to forward a letter to a missing person under circumstances involving a matter of great importance, such as a death or serious illness in the missing person's immediate family, or a sizeable amount of money that is due the missing person.
To find more ifno go to their main webpage: Social Security Online
Social Security Online
Do a search for 'Missing Relative' in the search box. The search should return 3 results. The first one will be about the letter forwarding service. This was the only way I found the information.
Best Wishes