If there is a garment that is stained with the blood of a deceased person, does that create an issue tamei meis inside the building the garment is housed in? (The blood having been spilled at the time of death, such as the shirt of a shooting victim, but long after the event so only the stain remains.)

ב"ה Shalom This question should only be in theory but not "lema'aseh" = practical. There are two questions here. One is: does the blood on the shirt create an issue of "tmei met" in a building and the other is does the shirt itself have to be buried? According to the Rambam ,( רמב"ם הלכות טומאת מת פרק ד הלכה יג) if there is a "revi'it" of blood absorbed in a garment then it creates a situation of "Tmei met". The Gesher Hachaim, vol. I pg. 76, quotes the Rambam as the Halacha. (How the blood is measured see Rambam ibid, and Tosefot Yom tov to Mishna Ohalot 3:2) If the blood on the shirt, came from the deceased Halacha distinguishes 3 situations. 1. When the blood came from a live person , even if he died some days later, but is was not the blood which came out just prior to death. This does not require burial even D'rabanan. שו"ת 'שבט הלוי' ( חלק ה סימן קעט) 2. דם הנפש- Blood which came out close to the death of the person or just before death, according to all opinions requires burial , D'rabanan. שו"ת 'חינוך בית יהודה' (סימן עה ,שו"ת 'חיים ביד' (להרב חיים פלאג'י סימן קא) 3. Blood which came out after death: there is a dispute among the Achronim. Rav Ya'acov Ruzha שליט"א , one of the leading experts on issues of Kevura, says therefore out of doubt that blood should be buried too. So therefore, blood remaining on a shirt if it came from categories 2 or 3 it requirs burial even long after the fact. Only according to the Gesher Hachaim, only if there is a Revi'it of blood in one place on the garment then it required burial. Besorot tovot