Thornton was on an American Airlines flight back in May from Charlotte to L.A. ... when a passenger eyed his laptop and saw him in a disturbing chatroom. The passenger tells TMZ, she saw images and texts that made it appear it was a chatroom for pedophiles.

One of the messages she saw said, "Couple of cute boys and a hot little girl." Another read, "All-you-can-eat Boyfett." referring to a restaurant where they could find underage kids.

And there's another message references "CP," which the passenger believed stood for child porn.

As a result, the passenger notified a flight attendant and shared these photos with the crew. We're told the captain then notified authorities, and law enforcement met the plane when it landed at LAX.

We're told authorities questioned Thornton and he was then free to leave.