The information about the lack of a cause of death in the case of Lyric and Lizzy came from the lead investigator in the murders of Abby Williams and Liberty German, Indiana State Police Lafayette District Investigative Commander First Sgt. Jerry Holeman who stated
"Obviously, the case in Iowa is probably the closest but they don't even know the cause of death in that one because they didn't find the victims until later so it's hard to compare, but it is similar, but, so, we take that into consideration as well. We, we have investigated that. We've been in contact with Iowa and they've been in contact with us and we've discussed that but nothing else seems to be popping up around the nation, er, similiar to this but doesn't mean he's going to kill everybody the same every time so we don't know."
http://fox59.com/2017/08/14/lead-de...ave-more-audio-from-teens-phone-dna-evidence/
The quote begins around 9:50.