That is actually a pretty low number. When a homicide is committed, you don't arrest because you don't know who it is, or you arrest someone because you do. For the most part, in a murder case, you can usually find the suspect unless they have fled the country which citizens rarely do, but illegals do often. Frankly, I'm surprise the statistic is not close to 100%
... no, it is not surprising that illegal aliens would kill without impunity here...but let us not profile (sarc off)