American Airlines Plane Forced to Land After 6 Fall Ill Mid-Flight A flight attendant "literally just fainted," one passenger said.

 -- A Los Angeles-bound American Airlines flight carrying 188 people was forced to return to London’s Heathrow Airport after several flight attendants and crew members fell ill mid-flight, the airline told ABC.

One flight attendant "literally just fainted... just fell forward, put her hand out and hit the floor," passenger Kris Evans told ABC News. "It was crazy."

The Boeing 777, which took off at 12:05 p.m., landed safely in London about five hours later, where it was met by ambulances, American Airlines said.

Paramedics examined, then discharged, six patients who complained of feeling unwell, according to the London Ambulance Service.

It was unclear what caused the patients to feel sick, but crews investigating the plane found no evidence of hazardous substances, according to the London Fire Brigade.

ABC News' Matt Hosford, Becky Perlow, Daniel Steinberger, and Rashid Haddou contributed to this report.