ANA flight's mid-air U-turn could result in criminal charges Two brothers got on the flight allegedly using a single boarding pass.

 -- The FBI is weighing criminal charges against two brothers who got on a Tokyo-bound flight together in Los Angeles allegedly using a single boarding pass, forcing the plane to turn around mid-flight.

Law enforcement sources with knowledge of the situation told ABC News that two brothers went through security at Los Angeles International Airport on Tuesday with legitimate tickets, but were booked on separate flights to Tokyo. One had a ticket on an All Nippon Airways flight and the other was booked on a United Airlines plane.

The brothers, who have nearly identical names, were able to board the ANA flight together by apparently using a duplicate boarding pass, the sources said. It's unclear how the brother with a United Airlines ticket found a seat on the plane. Their identities have not been revealed, other than being male American citizens.

The flight crew didn't become aware of the situation until roughly four hours into the flight. The airline then decided to turn around mid-air and head back to Los Angeles.

When the flight landed, the brothers were interviewed by authorities and released. There is no indication anyone was in danger.

"During the flight, the cabin crew became aware that one of the passengers boarded the incorrect flight and notified the pilot," ANA said in a statement. "As part of the airline’s security procedure, the pilot in command decided to return to the originating airport, where the passenger was disembarked."

"ANA is researching the situation currently to determine how the passenger boarded the flight," the airline added.

Among the hundreds of frustrated passengers aboard the ANA flight was model Chrissy Teigen and her husband, R&B superstar John Legend. Teigen, who has more than 9.2 million followers on Twitter, sent out a flurry of tweets Tuesday night when the plane made an abrupt U-turn on its way to Japan's capital.

By the end of the night, Teigen had sent more than two dozen tweets about the travel nightmare and racked up hundreds of thousands of likes and retweets. The 32-year-old model said she spent a total of eight hours and 20 minutes in the air, only to end up back at Los Angeles International Airport.

Newlyweds Devin McIntyre and Justine Williams, both of Washington, D.C., were embarking on their honeymoon after visiting relatives in Los Angeles. They both expressed disappointment that they lost a day from their honeymoon due to the airline's mistake, despite being compensated 30,000 yen, or about $265.

"You can’t make up a day. There’s no compensation that gives us back that day," McIntyre told ABC News after arriving at Tokyo International Airport on Wednesday. "And then the feeling of turning around in the middle of a flight doesn’t make sense. We tried to get the rationale on that decision, but nobody would give it to us."

The couple described the ordeal as a "horrible experience" and said they do not believe they were compensated enough because the sum "doesn't make up for the lost hotel and restaurant reservations and 24 hours of time in Tokyo."

"I mean, everybody on the flight was very nice. The crew and the people on the ground were all police. But, it seemed like there was a lack of leadership," McIntyre said. "Nobody was communicating, and they still need to step up and do the right thing."

FBI agents told ABC News on Wednesday night that they aren't in a position to arrest anyone or press charges yet, but the investigation is ongoing. If investigators determine there was intent to board the incorrect flight, one of the brothers could be accused of being a stowaway, which is a federal crime and punishable up to five years in prison.

Meanwhile, the airline could face hefty fines from the U.S. Customs and Border Protection for allowing the passenger on board and failing to make an accurate headcount.

ABC News' Karma Allen, Jeffrey Cook, Julia Jacobo and Michael Rothman contributed to this report.