No hostage killed by SWAT

In the August 30 issue of the Philippine Star, the news is that an examination of the shells showed that the hostages were killed by former police officer Rolando Mendoza.

http://www.philstar.com/Article.aspx?articleId=607445&publicationSubCategoryId=63





I don't get it. How can a forensic examination of the spent shells reveal that the hostage-taker killed the hostages? What I know is that the shells may reveal that they came from the hostage-taker's gun but they will never reveal if the slugs that came from those shells killed the hostages. The slugs themselves will determine that and not the shells. No wonder the Philippine police is not exactly respected for its investigative techniques.

from JP