A man who had missed his flight Wednesday said there was a bomb aboard the plane, forcing it to return to Seattle-Tacoma International Airport even though authorities did not believe the threat was legitimate, an airport spokeswoman said.

"The plane was brought back, erring on the side of caution, after he made that threatening statement," Transportation Security Administration spokeswoman Jennifer Peppin said.

Police confirmed there was no bomb on board hours later, after bomb-sniffing dogs searched the plane, airport spokeswoman Terri-Ann Betancourt said.

The man told a gate agent there was a bomb on Northwest Airlines Flight 980 shortly after the plane left Seattle for Memphis around 12:33 p.m., Betancourt said.

He was taken to a holding facility at the airport police department's headquarters.

"After interviewing him, they said they do not believe it was a credible bomb threat," Betancourt said.

The man was never on the plane and had no luggage on the aircraft, Betancourt said. She said the man's motive was unclear, but "it was clear he wanted the plane to come back."

The plane, an Airbus A320 with 142 passengers and 5 crew members on board, returned to the airport around 1:15 p.m., Betancourt said.

Passengers remained on the plane for more than an hour while police tried to determine how extensive a security sweep was needed. Everyone was taken off along with all luggage, and bomb-sniffing dogs searched for any traces of explosives, Betancourt said.

Passengers were given the option of rebooking if they did not feel comfortable getting back on the plane, Northwest spokesman Roman Blahoski said. The plane finally departed for Memphis — for the second time — at 5:14 p.m., Blahoski said. It reached its destination late Wednesday.

Betancourt noted it's a federal offense to make a threat against a commercial aircraft but said she could not release any information about the man in custody.

Police and the FBI were investigating.