When flight attendant Sheila Fedrick spotted a dishevelled girl on an Alaska Airlines flight from Seattle to San Francisco, she knew something was wrong.
The teenager "looked like she had been through pure hell", the flight attendant told NBC and the well-dressed man she was travelling with would not allow the girl to be spoken to.
Fedrick tried to engage them in conversation, but the man became defensive, she said.
On instinct, the Fedrick left a note for the girl in one of the bathrooms on board the aircraft.
"I left a note in one of the bathrooms," Fedrick said. "She wrote back on the note and said 'I need help.'"
Fedrick informed the pilot of the passengers and the police were waiting to escort them once the plane landed.
Fedrick's instincts were correct - the girl was a human trafficking victim.
The 2011 incident was reported in US media this week, as charity Airline Ambassadors seeks to train airline staff in ways to combat human trafficking.
At an Airline Ambassadors training session this week, crews were taught to look for passengers who appear unsure of their destination or nervous; children travelling with someone who doesn't appear to be a parent or relative; and children or adults who appear bruised or battered.
They're also taught to notice if someone insists on speaking for the alleged victim, doesn't let them out of their sight or becomes defensive when questioned.