Thank you for your question.
"In the event of a death it is important to contact certain institutions:
Any public or private institution for that the deceased worked for or received payment from
The Director General of the Public and Personal Pensions(Dirección General de Costes de Personas y Pensiones Públicas), or the Institute for Social Security (Instituto Nacional de la Seguridad Social INSS) as appropriate. Pension payment can then be altered accordingly, to widows pension or orphan allowance for example
Banks and insurance companies (life insurance). Banks often insist that an accident policy is taken out when an account is opened
To find out if the deceased made a will or to obtain a copy of the will, contact the Registry of Last Wills
of the Ministry of Justice, (Registro de Últimas Voluntades del Ministerio de Justicia)"
That page links to the Director General of Pensions, and that page links to Social Security.
You should not have to go to Spain, but you will need to provide a certified copy of the death certificate, The death certificate must be certified and probably has to be apostilled because although Spain is a party to the relevant Hague convention, the U.S. is not, see
The office that issues the death certificate should be able to arrange for it to be apostilled.
I hope this information is helpful.