American Airlines flight AA109, headed to Los Angeles, was forced to turn back to London after several crew members and passengers became mysteriously ill.

The plane was about 1,600 kilometres out from Heathrow Airport when it turned back. Upon arrival, everyone’s luggage was seized for checks.

The Daily Mail reports that one member of the cabin crew fainted and up to three other passengers, including a man believed to be in his 60s and another in his 40s, complained feeling unwell.

Upon arrival, passengers’ luggage were confiscated and checked by American Airlines and Heathrow authorities, but passengers were kept in the dark with what is happening.

The airline has not released a statement regarding the flight turnaround incident or what caused the people on board to feel unwell.

Passengers reported on Twitter that officials boarded the plane to test the air before they were instructed to disembark.

Yahoo News! also relays that one of the passengers named Alan Gray told The Daily Mail that he saw one of the flight attendants collapse while other passengers on board looked “ill” and “iffy.”

“One of the flight attendants was walking down the aisle when she collapsed. Then up to six other cabin crew members said they felt light-headed and as though they were going to faint,” Gray said before the captain announced he was turning the plane around.

Gray said that upon landing in London there were police cars, ambulances and firefighters to escort them off the runway, after which they were held on the tarmac for 45 minutes.

“Eventually, when we did dock, there was only one person who came on and he was monitoring the air,” he said.

A spokesperson from American Airlines confirmed that the plane had turned around after a medical emergency where the cabin crew made the decision to fly 1,610km back to London.

“We apologise to our customers for the inconvenience to their travel plans,” the spokesperson said. All luggage was returned to passengers.