Embassy of Latvia

Latvia has put all church books online. http://www.ciltskoki.lv/index.php/lv/raduraksti (Baznīcu grāmatas) If you know the town in Zemgale, look up those books.

You have to make an appointment on-line on the embassy website, and bring with you a pre-filled form for a passport renewal request, a declaration of your place of residence and your expired Latvian passport. You can download both froms from the website. You also have to bring a proof of legal status in the U.S. - greencard, U.S. passport (if dual citizen), valid U.S. visa etc. They will not renew your passport without the last. Make sure you have a confirmed appointment - no walk-ins are accepted. Print your appointment confirmation and bring with you. New passport can take few months to receive. Good luck!

I saw info on Facebook page