A Los Angeles-bound American Airlines flight was forced to return to London's Heathrow Airport Wednesday after a mysterious illness spread through the jet that caused at least six people onboard to fall ill.

American Airlines Flight 109 took off from Heathrow at around 12:05 p.m. as it departed for Los Angeles, reported CBS News. However once in the air, two passengers and some of the flight attendants complained of light-headedness. Witnesses reported that two people fainted and one person vomited on himself.

As news of the illness began to circulate throughout the cabin, the pilot spoke to the crew asking if any doctors, nurses or medical professionals available to help.

"About 2.5 hours into the flight just as we were passing Iceland we had a Tannoy announcement asking for any doctors, nurses or medical professionals on board to report to the boarding doors to assist with unwell passengers," passenger Lee Gunn told The Daily Mirror. "The lights then came on in the cabin and there was lots of commotion."

The pilot made a second announcement shortly afterward, revealing that he decided to turn the plane around and return back to London, rather than risk continuing the flight.

"The pilot came on and spoke to everyone and said, 'Listen, the three of us are safe up front. We have zero problems, we're taking us back to London,'" passenger Eric Winter recalled.

Teams of paramedics and fire crews were lying in wait of the aircraft after it made its return-landing, who came onboard to evaluate the passengers, reported The Telegraph. No one other than those sickened while in the air needed additional help, and were released from the scene.

American Airlines later confirmed the incident in a statement, calling it a "medical emergency." It added that a hazmat team checked the plane and all the luggage for "elevated levels of substances," but all tests turned up negative. Regardless, the aircraft is being thoroughly cleaned to be on the safe side.