Prosecutors Waiting in Gabby Petito Homicide, Loose Ends Involving Brian Laundrie

Gabby Petito's death has put her fiance, Brian Laundrie, in the crosshairs of law enforcement, but TMZ has learned prosecutors are not making a move until 2 primary loose ends are tied up.

Law enforcement sources with direct knowledge tell TMZ ... Laundrie is their suspect, despite the fact he's being officially called a person of interest. That said, we're told federal prosecutors are now in a waiting game, in large part because they don't know if Laundrie is still alive.

What's more ... our law enforcement sources tell TMZ, even if Gabby was murdered -- which seems to be the case since the Coroner has already ruled her death a homicide -- they would need to prove who did it. Our sources point out, it's possible Laundrie left Gabby out in the wilderness and someone else killed her ... whether it's probable or not, it's still possible.

As for why federal prosecutors will handle the case, it's because Gabby was apparently killed in the Grand Teton National Park, which is federal land.

We're told law enforcement is viewing Laundrie as their prime suspect, but there's a leap between that and actually charging someone with murder.

As we reported, a witness called 911 and told the dispatcher Laundrie had slapped and hit Gabby shortly before she went missing.