Over the summer, the couple embarked on a cross-country road trip, from which Laundrie returned to their North Port, Florida, home alone on September 1, police say.

Laundrie, without talking to authorities during Petito's disappearance, left his home September 13 and was not seen again, his parents later told police.

"Two people went on a trip, and one person returned," North Port Police chief Todd Garrison said in September.

With Laundrie now gone and many questions still unanswered, here's what experts say investigators could home in on to help them answer what happened to Petito.

Crime scenes will be critical in investigation

Among the most important components in an investigation like this are the crime scenes, said Paul Belli, a retired lieutenant of the Sacramento County Sheriff's Office and president of the International Homicide Investigators Association.

Investigators also obtained a search warrant last month for an external hard drive they found in Petito's white van -- the one the couple used during their trip and in which Laundrie returned, alone, to their North Port home.

Finding phones in a case like this could also offer an "incredible amount of information," Belli said.

A source close to the Laundrie family told CNN last month Laundrie left home without his wallet and without a cell phone he had purchased earlier in September.

Police body cam footage showed Petito crying uncontrollably as she talked to police.

"We have someone who did not report his fiancée missing when he returned without her, we have someone that fled," Rahmani added.

And in an interview Thursday, Steven Bertolino, the Laundrie family attorney, said Laundrie was "grieving" and appeared upset when he left his family's home in mid-September.

Laundrie's parents knew their son was "grieving, they knew that he was so upset and, you know, they just couldn't control that he was leaving and he left,'" Bertolino told CNN affiliate WABC.