If you're asking if there is a goverment agency that can assist people who are searching lost relatives in general, the answer is no (I'm just curious. Is there one in your country? Because I'd like to search about a Japanese relative who died in Vagaville, California).
City halls can look up on registeries (koseki) for those with names and home addresses. The government is also working on special issues like the ex-orphans who were left behind in China upon the ending of WW2. But your case wouldn't fit.
But of course, you can try asking for advise at city halls of places you might think that are related to this matter (Hiroshima?). The city halls are there to give people advise.