Mystery Illness That Grounded American Airlines Flight Remains Unsolved

An illness that led an American Airlines pilot to abort a flight Wednesday remains a mystery a day after the incident.

The pilot captaining American Airlines Flight 109 from Los Angeles to London decided to return the plane to its origin just hours after it departed when a member of the crew fainted and five others complained that they felt sick, according to The Telegraph. The Boeing 777 plane landed safely and emergency health workers met passengers on the ground.

“Our maintenance team conducted a thorough inspection of the aircraft, including a test flight, and found no issues,” the airline said in a statement. The air filters were replaced “out of an abundance of caution,” an airline spokesperson said.

Some reports have suggested that the illness may have been caused by air leaking from the plane’s engine into the cabin. Such leakage has been alleged for years but never proven to be toxic. The issue is the topic of pending litigation by Alaska Airlines flight attendants who passed out on a flight in 2013. The flight attendants say that Boeing—the manufacturer of the plane—was aware of a defect that allowed toxic air into the cabin; Boeing says the air is not harmful.

Write to Justin Worland at justin.worland@time.com.