LONDON — American Airlines Flight 109, traveling to Los Angeles from London, was more than two hours into its journey and close to Keflavik, Iceland, when several passengers and crew members suddenly and mysteriously became ill. Those aboard were startled, news reports said, and the pilot decided to fly back to London.

The plane landed safely. But on Thursday, the mystery of what happened on the plane, which captured headlines in Britain and beyond, remained unclear. The Daily Telegraph in Britain, citing a passenger on the flight, reported that one crew member had fainted.

American Airlines said in an emailed statement that two passengers and several flight attendants had complained of lightheadedness during the flight on Wednesday, prompting the captain to return to Heathrow Airport near London. It said there were 172 passengers and 16 crew members on the Boeing 777, which landed at 5:05 p.m. News reports said the plane had been met by paramedics, police cars and firefighters.

Passengers and crew members were evaluated by health officials, American Airlines said, but none requested further medical attention. Luggage was removed for inspection, The Daily Telegraph reported.