A British Airways plane was held for security searches for three hours, after landing at Washington's Dulles airport on New Year's Eve.
Passengers had to wait three hours before they could disembark
The Boeing 747 arrived on Wednesday evening, with 247 passengers and 17 crew on board.
An FBI spokeswoman said some passengers
were questioned but no arrests made.
The US has raised its national alert status to orange - the second-highest level - and six Air France flights to Los Angeles were cancelled last week.
Flight BA 223 from London Heathrow landed at Dulles at 1906 EST (0006 GMT Thursday).
It was held in an area well away from the main terminal building, while officials from the FBI and the Transportation Security Agency boarded the plane.
Officials began allowing passengers off the plane at around 1030 EST (0330 GMT Thursday).
Federal Bureau of Investigation spokeswoman Debbie Wierman said
some of the passengers "were folks that law enforcement officers
were interested in speaking with."
But it was described as "routine" questioning with no one arrested.
"We do not know what triggered this investigation,"
said a BA spokesman.
It was over three hours before passengers were ferried to the arrivals building.
Officials did not say why the flight had been singled out for checks.
A spokeswoman from the Department for Homeland Security would only say the plane had been "detained... for further screening".
Meanwhile, US intelligence had warned of a "credible threat" from al-Qaeda operatives due to board a flight from Paris to Los Angeles.
French intelligence questioned several people booked on the suspended flights, but said they had found no links to militants, and flights were restarted.