DETROIT (Reuters) - When the first emergency alert from Detroit’s airport went out just before noon on Sunday, it looked oddly like a mistaken repeat of the scare from two days earlier: “Nigerian national caused disturbance on Flight 253.”

As the Airbus 330 approached Detroit after a nine-hour flight from Amsterdam, a Nigerian who was sitting in the plane’s last row locked himself in the bathroom and refused to come out even when the flight crew ordered him to do so.

On Friday, another Nigerian man, Umar Farouk Abdulmutallab, was accused of trying to ignite an explosive on the same Delta Airlines flight to Detroit.

“My first reaction was that someone had hit the send button on the same message by mistake,” Michael Conway, a spokesman for Detroit Metro Airport, recalled thinking when he saw the first emergency alert on a pager.

After Friday’s incident, U.S.-bound passengers have faced tighter security measures, including pat-downs and an order to stay seated during the last hour of their flight.

The Nigerian man on Sunday’s flight, who was not identified, was released after FBI agents determined he was genuinely sick and not involved in any plot against the plane.

But passengers among the 275 other people on board faced a tense wait while officials conducted interviews, bomb-sniffing dogs pored over luggage emptied onto an icy tarmac and police cordoned off the plane on a remote part of the airport.

After landing in Detroit, the Nigerian man was led out in handcuffs by FBI agents while police with automatic weapons surrounded the Northwest plane, passengers said.

“Everybody was scared, freaked out. We were all just shaking,” said Niranjanar Kumar, who was flying back to his home in Michigan after a visit to Delhi.

Other passengers said they had no indication of anything unusual until the plane approached Detroit.

At that point, two undercover air marshals identified themselves and went to the rear of the plane to join the flight crew outside the bathroom, several passengers said.

Passengers were taken in six buses to a large waiting area of the Detroit airport and barred from using cellphones while all the baggage was searched. Some of the travelers were interviewed.

“It took four hours for them to decide what to do with us,” said Ken Raub, a Detroit-area resident returning from Moscow where he and his wife adopted a baby girl. “It was a little rough.”

FBI agents were still at the Detroit airport as part of a continuing investigation of the Christmas Day incident and joined customs, border patrol agents and airport police and fire crews in responding to the scare, officials said.

One flight attendant appeared to faint and was treated on the plane after it landed, said Jane Jeronimus, an Australian passenger who sat in business class.

Hitesh Desai, who was traveling from Bombay to Detroit, said he was unaware that anything was unusual until the plane landed and he saw all the police waiting.

“This was just a normal flight,” he said. “It did feel kind of surreal while it was happening.”