Brian Laundrie, who authorities had said could help fill in at least some of the blanks about what Gabby Petito's final days looked like, has been confirmed dead.
Mr Laundrie's remains were found Wednesday in the Florida reserve authorities had been combing through for more than a month.
Over the summer, the couple embarked on a cross-country road trip, from which Mr Laundrie returned to their North Port, Florida, home alone on September 1, police say.
READ MORE: Police say timeline of Brian Laundrie's disappearance not adding up
On September 11, Ms Petito's parents reported her missing.
Mr Laundrie, without talking to authorities during Ms Petito's disappearance, left his home September 13 and was not seen again, his parents later told police.
The young woman's remains were recovered in Wyoming on September 19. A coroner ruled she died by strangulation.
But little else is known – at least to the public – about what led up to Ms Petito's death.
Though authorities have not explicitly connected Mr Laundrie to Ms Petito's death, they have said he was among the last people to see her alive.
"Two people went on a trip, and one person returned," North Port Police chief Todd Garrison said in September.
With Mr 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 Ms Petito.
READ MORE: Gabby Petito was strangled, coroner says
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.
Teton County, Wyoming, Coroner Dr Brent Blue said Ms Petito died by manual strangulation/throttling, adding that her body had likely been outside in the wilderness for about three to four weeks before being found.
"Throttling means that someone was strangled by human force. There was no mechanical force involved," Blue told CNN.
READ MORE: New bodycam footage of Gabby Petito unearths dispute with fiancé
There could be clues as to what happened – and who did it – that investigators may be able to collect from a crime scene, Belli said, including fingerprints, depending on the condition of the human remains.
What's difficult in cases like these is that unlike killings that are committed by strangers of the victims, when the person of interest is someone the victim was involved with, finding their DNA on a victim's body is to be expected.
"You would expect DNA on either one of them from the other," Belli said. "But I mean, if there's DNA maybe where it shouldn't be, that could be kind of a clue as to what may have occurred."
"So there are ways to absolutely, at least get you to a point where you're like, 'okay, this definitely makes sense, this is the person who did this crime,'" he added.The type of crime also can help offer hints, said former federal prosecutor Neama Rahmani.
Killings involving manual strangulation can often indicate "an emotional element," Rahmani said.
Some key items could provide answers
Investigators last week recovered personal items such as a backpack and notebook along with Mr Laundrie's remains. A source with knowledge of the investigation told CNN's Randi Kaye the notebook is "possibly salvageable."
And experts say that could offer more insight.
Those items will likely be taken to an FBI lab where there are "experts who really spend their careers doing things like drying out paper evidence, trying to recover the writing and the ink marks and potentially fingerprints and all sorts of other potentially relevant pieces of evidence from an article just like this," former FBI Deputy Director and CNN senior law enforcement analyst Andrew McCabe told CNN's Wolf Blitzer Thursday.
The notebook could potentially include information like Mr Laundrie's motives, his feelings about Ms Petito and other notes he made about events in Wyoming, Mr McCabe added.
Investigators also obtained a search warrant last month for an external hard drive they found in Ms Petito's white van – the one the couple used during their trip and in which Mr Laundrie returned, alone, to their North Port home.
Authorities have not shared details on what they found on the drive.
Finding phones in a case like this could also offer an "incredible amount of information," Belli said. A source close to the Mr Laundrie family told CNN last month Mr Laundrie left home without his wallet and without a cell phone he had purchased earlier in September.
Police do not have the phone he had with him during the couple's trip, nor do they have Ms Petito's phone, CNN confirmed earlier in October.
Finally, videos that authorities have collected of Mr Laundrie and Ms Petito could also help put together pieces of the puzzle in the couple's journey – and perhaps Mr Laundrie's journey back to Florida.
"I don't think people realise the sheer volume of information that we now get on every case," Belli said.
"Video, phone records… if that vehicle had any information that can be gleaned from it with a GPS."
Mr Laundrie was charged with using a debit card and PIN for accounts that did not belong to him after Ms Petito's death, according to an indictment.
"(Authorities) probably collected video from wherever those were used, video for wherever he may have appeared to stop for a period of time. I doubt he drove completely straight through," Belli said.
"So, there's a lot of additional work to be done that has been in progress, most likely."
Circumstances may help unravel the mystery
In a case like this, circumstances alone can also help paint a clear picture, Rahmani, the former prosecutor, said.
"It's a very, very strong circumstantial case," he said. "You have a history of violence between the two.
"You have all evidence that indicates (Laundrie) was the last person to see her alive and the manner of death, that manual strangulation, that tends to be… most often someone you know."
In August, Utah authorities had an encounter with Mr Laundrie and Ms Petito and described them as having "engaged in some sort of altercation."
The two were described as getting into a physical fight following an argument but both reported "they are in love and engaged to be married and desperately didn't wish to see anyone charged with a crime," one officer's report stated.
Police body cam footage showed Ms 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 Mr Laundrie family attorney, said Laundrie was "grieving" and appeared upset when he left his family's home in mid-September.
Mr 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.
Experts have questioned what the Mr Laundrie family may know about what happened to Ms Petito.
"Did they help him escape? Did they help destroy evidence?" Palm Beach County, Florida, State Attorney Dave Aronberg told CNN's Wolf Blitzer Friday night.
"What did they know?"
Though with Mr Laundrie deceased it may be harder to find the answers that both investigators and the families were hoping for, there may still be a lot that could be done in the investigation into Ms Petito's killing, Belli said.
He said this could remain an open case for some time if authorities are not able to confidently say they know who killed her.
"I mean the closure is really, did he do it or did he not do it, that is going to be the overarching feeling based on my experience," he said.
"The investigators," he added, "I guarantee that they feel a great need to provide the truth, whatever that truth is, to both sides of the family. That's really what we do as investigators is find all the facts and lay out the truth."
