A flight from Chicago to Hong Kong had to turn back after a passenger began smearing faeces on the walls.
United Airlines Flight 895 was diverted to Anchorage, Alaska on Thursday night (local time) due to a "disruptive passenger", the airline said in a statement.
But police in Anchorage told local media the real reason: a passenger was wiping the toilet walls with faeces and tried to stuff his shirt down a toilet.
Another passenger told network KTVA he was "shaking the seats all around him… pouring soda and water on the floor and on laptops".
"Something wasn't right with him upstairs," they said.
FBI agents and police apprehended the man on landing, and he was taken to hospital for a psychiatric evaluation.
No charges have yet been filed against the man, whom two days earlier had been released from prison in Indiana. He had a one-way ticket to Vietnam, where family members live.
Other passengers were provided with accommodation and new flights, according to KTVA.