If you are looking for information about family members who participated in the Second World War, there are several memory institutions where you can find it. Last spring I organised a webinar with the Latvian War Museum historian Jānis Tomaševskis. This blog post is a short summary with the information he provided in the webinar. The lecture was held in Latvian and a video recording can be viewed here. At the moment, it is not translated into English.
In Latvia, the Latvian War Museum, the Latvian State Historical Archive , and the Latvian State Archives (both part of Latvian national Archives) hold information about Latvian citizens who served in the German and Soviet military units during the war. The German Federal Archives (Bundesarchiv/Deutsche Dienstelle) and the Arolsen Archives are other institutions where information can be found.
The Saxon Memorial Foundation holds information on Soviet prisoners of war in Germany, while the lists of prisoners of the British and US armies in POW camps are held in the Latvian War Museum (for the Zedelgem camp), in the Latvian State Archives (the collection of the Hoover Institution), and the relevant foreign (British and US) archives.
Before requesting information from any of these institutions, it is recommended that you do some research first and gather as much information as possible about the person you are searching for. The minimum required information is the name and date of birth, but any additional information about the person's service will be useful. Some of these authorities may require proof of relationship in order to obtain information about the person.