HOUSTON -- An investigation is under way to determine why a mental health patient was traveling alone on a Continental Airlines flight from Houston to Omaha, Neb.

Passengers said the man left the plane's bathroom covered in his own waste on the Dec. 26 flight, reported KPRC-TV in Houston.

The commuter jet had one flight attendant, who moved passengers forward to empty seats and kept the unkempt passenger in the back row.

Stacey, a passenger on the plane who requested her last name not be revealed, said the man attacked the flight attendant.

"I hear all of this ruckus and yelling," she said. "I turned around and the poor flight attendant is on his back and the guy is, like, punching him."

Stacey said other passengers came to the flight attendant's aid.

"It's almost like a scene out of a movie," Stacey said. "There were two male passengers behind me that got up and kind of got the guy off of him. The poor steward, he got a black eye."

The man was not arrested but was turned over to mental health professionals from a care center in Iowa where he reportedly lives.

The U.S. attorney's office said proving criminal intent could be difficult because of the man's mental state. The FBI will investigate how the man was allowed to fly unsupervised. He was taking a holiday trip from Chicago to Houston to Omaha.