How can I file a complaint of elder abuse of a relative? He was a US citizen, I do not live in the United States

I think that your allegations would be impossible to prove from a distance and after the fact - since he is deceased what would you hope to accomplish?

Contact either the city, county or state in which your relative lived and ask how to reach the Adult Protective Service  or other agency governing and monitoring elder abuse.  Or just google the city, county, township or state, plus APS. 

You could also contact the local police department in the area where your relative lived.

If the alleged abuse was financial, make sure that you have records and data to provide to an investigator.  This could be much more difficult in that your relative is no longer here to be examined.   

I am sorry for your loss; it must be so much harder to deal with when you're in a different country.

I agree with CWillie, that your allegations could not be proven now that your relative is dead. Just possibly, if fraud has been perpetrated involving and elder and his estate there could be a legal case, but that would be almost impossible for you to do from outside the country. It, of course, certainly is worth reporting. Be prepared to fax your evidence to Adult Protective Services in your area. I won't be surprised if they refuse to file a case on a deceased person, but also won't be flummoxed if they DO, so worth a try to protect someone in future.