3:31 PM — Law enforcement sources connected to the search for Travis Decker tell TMZ … the belief is he probably killed himself and is lying dead somewhere.

Our sources say it’s hard to find someone in a forest when they’re not mobile … because there’s no trail for authorities to pick up on … and if Travis is dead, it’s going to be hard to find his body.

Still, we’re told the investigation remains open and the possibility still exists that Decker is alive somewhere.

Our sources say sometimes people can be gone and stay gone, but it’s hard to stay gone in this day and age, especially for months … as has been the case here.

False alarm in the ongoing manhunt for suspected killer Travis Decker … the feds say bones found near where his dead daughters’ bodies were discovered turned out to be from an animal.

The FBI says Central Washington University examined the bones and determined they are not from a human.

As we reported … the FBI said last week they found bones near the Leavenworth, WA campground where Decker’s daughters — Paityn, 9; Evelyn, 8; and Olivia, 5 — were found dead.

Decker is the main suspect in the girls’ deaths … his DNA — and only his DNA — was found on the plastic bags covering his late children’s heads. The county medical examiner ruled they died by suffocation and their deaths were pronounced homicides.

The FBI says search teams scoured almost 250 acres near where the bodies were found … but Decker’s whereabouts remain a mystery.

When the bones were found, people thought it may mean Decker had been dead for a while … but they aren’t his remains.

Law enforcement has been hunting for Decker since his girls were found dead back in June … his truck was found not long after … 150 miles outside of Seattle and in a heavily forested area.

Some folks theorize Decker may have fled into the wilderness and is using his military background as a way to possibly escape capture.