Larie Pidgeon tells TMZ ... she knows LAPD detectives "worked tirelessly, and their efforts have brought us to this new piece of the puzzle. But I also want to be clear -- this search is not over. Knowing Hannah crossed the border does not provide the answers I need, nor does it ease the heartbreak I feel."

Larie's reacting to an image LAPD's Missing Persons Unit says it has of Hannah willfully crossing the border into Mexico on Nov. 12. Hannah's family reported her missing about 2 weeks ago, after she missed a scheduled flight to NYC from LAX on Nov. 8, the last day she was captured on camera.

But, her aunt still suspects something nefarious is going on, telling us ... "Hannah never mentioned any plans to travel to Mexico, and no one in her life knew she intended to go there. What alarms me even more is her complete disconnection from her phone, her social media, and her world -- this is not who she is."

Larie says it's been 21 days since she last heard from Hannah, and she adds ... "I can’t shake the last messages she sent friends and family, which I’ve replayed in my mind a thousand times, trying to make sense of it but still cannot."

The texts to which she's referring were sent from Hannah to her mother, and also to a friend, on Nov. 11 ... and she informs them she didn't make it to NYC. She also says something about meeting a guy she'd first met through the internet, and running out of money.

Larie's thanking the public for all their help up to this point, but says she still needs their support through this "most painful and confusing time of my life."

She adds, "I will not stop until I know my niece is safe and doing this on her own accord and in a good mental state."