The United Airlines flight rerouted to Anchorage after the incident, in which the passenger also allegedly attempted to flush his shirt down a toilet, according to officials.
The man, whose name was not released, spread his excrement around the toilet facilities on Flight 895.
FBI spokeswoman Staci Feger-Pellessier said there were no indications the incident was terror-related, and no injuries were reported.
Ms Feger-Pellessier had initially stated that "an unruly passenger had caused some kind of disturbance" on the plane.
According to Anchorage airport police lieutenant Joe Gamache, the man did not attempt to interfere with the flight crew or resist them, and was not expected to be arrested.
"We received a report of a passenger who had messed up the bathrooms with his own faeces," said Lt Gamache.
Upon landing the man, who was shirtless, was interviewed.
"After the interviews were done, there were no appropriate charges for anything criminal," Lt Gamache said.
A translator was needed to interview the 22-year-old, who was travelling on a Vietnamese passport and possessed a US permanent resident card.
Lt Gamache said that the man's speech was either nonsensical or unintelligible to the translator.
He has now been taken to a hospital for a mental health assessment.
Hotel accommodation was provided for the other passengers on the flight, said United Airlines.