There was a ton of speculation that because the mom and dad had drug issues that it was likely that the reasoning behind the girls disappearance. But police have never said that, and the profile the FBI released on the likely suspect pretty much refutes it entirely.
(Copypasta from the BAU profile)
This profile was created with help from the FBI's Behavioral Analysis Unit, and concluded that the alleged culprit of Lyric and Elizabeth was:
- A local that blended in with the Evansdale or Black Hawk County community, who acted upon an opportunity to abduct the two girls.
- Someone that was undoubtedly familiar with the Meyers Lake/Angel Park area, as well as Seven Bridges Park - meaning that they are unlikely to have been a transient or a wanderer. Police insist that the location where the bodies were found - Seven Bridges Park - was without a doubt familiar to the killer, as it is still relatively isolated and not well-known outside of those from the region, indicating that the killer is - or was - a local.
- Someone who would use "quiet coercion" to convince the two girls to go with him or her: either a trick or ruse of sorts, or some kind of threat of violence.
- Someone who was likely facing significant stress in their life (legal issues, mental health problems, issues with a spouse, etc.), who decided to lash out at two strangers instead of those close to them.
- Either someone who followed along closely with the investigation - or someone that tried to distance themselves as much as possible.
- Someone that had likely attempted abductions in the past, and had likely expressed interest in underage girls.
- And, finally, they were likely someone that may have attempted to alter their own physical appearance as well as the appearance of their vehicle in the immediate aftermath, which - again - was in July of 2012.