What I found interested was the following:
In a news release sent Saturday evening, an FBI spokeswoman said Mariahs remains had been found in Shelter Creek along Shaw Highway in Pender County, North Carolina.
Officials did not say what led them to arrest Kimrey, or what evidence suggested that Mariah was no longer alive.
[...]
Mariah was presumed dead based on evidence gathered throughout the weeklong investigation into her disappearance, according to the Onslow County Sheriffs Office. At a news conference on Saturday, FBI officials called the search for Mariah a homicide investigation.
http://www.newsobserver.com/news/local/article187931874.html
to me this suggests they strongly felt she was not just dead, but a murder victim. Even before or as they found little one's body. They may not be able to proceed to charge anyone with that murder until autopsy report but I think they have a much better idea what happened then they have stated and are putting all the little duckies in a row.