Chrissy Teigen appears during a State Dinner at the White House in Washington, Wednesday, March 14, 2012. (Susan Walsh/AP)

A Tokyo-bound flight made a U-turn while flying over the Pacific Ocean and returned to Los Angeles because one passenger, for reasons still unclear, had boarded the wrong flight.

All Nippon Airways Flight 175 left the Los Angeles International Airport at about 11:30 a.m. Tuesday Pacific Standard Time, spent about eight hours in the air and landed back in California just after 7:30 p.m., majorly inconveniencing its passengers, including model Chrissy Teigen, who live-tweeted the “flight to nowhere” to her 9.2 million followers.

The incident involved two brothers, one of whom was an authorized All Nippon Airways, or ANA, passengerwhile the other had a United Airlines ticket, Transportation Security Administration spokesman Mike England said.

The brothers, who were not identified, went through security screening without incident and had valid passports and boarding passes. Both were headed to Tokyo and were scheduled to leave around the same time, England said.

Both ended up on the ANA flight even though only one of them was an authorized passenger, England said. It remains unclear how that happened.

United and ANA are code-share partners, meaning one airline can sell seats on flights operated by the other.

The incident is now under investigation by FBI agents, who have talked to both brothers, England said.

All Nippon Airways said in a statement that the pilot flew the plane back to the Los Angeles airport after the cabin crew discovered that one of the passengers was not supposed to be there. The airline said the decision to turn around was part of a security procedure.

“ANA is researching the situation currently to determine how the passenger boarded the flight. ANA would like to express its apologies to the passengers for the inconvenience,” the airline said.

In a follow-up statement, the airline repeated its apologies, saying it “failed to deliver the customer service we strive for and passengers expect from us.”

Teigen, who was traveling with husband John Legend, wrote on Twitter that the plane was turned around about four hours into the flight. The Twitter world watched in amusement as Teigen kept her followers updated on the ordeal, which apparently involved an hours-long wait at the airport, unanswered questions, some police presence and a microwaveable Ramen meal.

Teigen and Legend appeared to have left Los Angeles on a rescheduled flight by early Wednesday morning. FlightAware, an online flight tracker, shows the flight will arrive in Tokyo a little after 7 a.m. Thursday, Japan Standard Time.

The Los Angeles Airport Police’s media office has not returned a call seeking comment, but Lt. Adonis Cutchlow told the Los Angeles Times that there was no illegal activity on board and referred further questions to the airline.

Weather concerns, mechanical problems, medical issues and terrorist threats are among the common reasons for diverting a plane. Others are unusual, but not unheard of.

In 2015, for example, a British Airways flight flew back to Heathrow Airport just 30 minutes into the seven-hour flight to Dubai because of an overwhelming odor coming from the lavatory.

A similar incident happened earlier this year, when a SpiceJet flight headed to New Delhi was diverted because of a “very foul smell” coming from the lavatory.

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