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 e-mailed 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 attention.

The aircraft was inspected by maintenance workers upon arrival, the company said, but it declined to elaborate. “We apologize to our customers for the inconvenience,’’ it said.

Although it remained unclear what had caused people on board to become ill, dizziness and loss of consciousness are among a range of symptoms that have been associated with jet engine fumes leaking into the cabin.

Modern commercial jets are designed to siphon heated air, drawn from the engines, for use in cabin pressurization and air conditioning systems. Occasionally, that air can become contaminated with traces of engine oil or hydraulic fluid, provoking acute symptoms, sometimes called aerotoxic syndrome, that eventually subside.

Airlines and regulators closely monitor such risks, but some pilots’ and flight attendants’ unions worry about the health risks of repeated exposure to toxic substances.