SEATTLE -- The FBI is investigating an allegation of a sexual assault on board a Northwest Airlines flight.

An employee of Northwest Airlines was taken into FBI custody on Monday morning after the alleged incident on board an overnight flight from Seattle to Minneapolis.

A 22-year-old woman told her boyfriend she was part way through the flight when a man sat down between her and another female passenger. She said she felt her shirt being lifted, then the man touched himself and left.

She said when she pointed out the man to a flight attendant, she was told he was a Northwest Airlines employee.

The woman's boyfriend told KIRO 7 Eyewitness News he is outraged that she was assaulted on a plane full of people.

"I know she was really upset and confused about what's going on, what's happening, (and) kind of scared," said Mark, who did not want his last name used.

The FBI met the plane in Minneapolis and took the man into custody.

The man, 20-year-old Samual Oscar Gonzalez is from Lakewood, the FBI said. He made an appearance in federal court Monday afternoon and was detained pending further court proceedings.

Northwest Airlines issued a statement saying Gonzalez was involved in an incident with another passenger and has been suspended pending a review of what happened.

The airline said it's cooperating fully with law enforcement authorities.