A human fetus was found on an American Airlines flight on Tuesday morning.

American Airlines spokesperson Justin Franco told TIME that flight 1429 came in to La Guardia Airport from Charlotte, N. C., Monday and was parked overnight. A cleaner readying the flight so it could go back to Charlotte on Tuesday morning discovered the fetus, Franco said.

The cleaner notified a supervisor, who contacted law enforcement.

The New York City Chief Medical Examiner’s office is investigating the fetus to determine exactly how it died, a spokesperson told TIME.

Franco said the airline is “working with law enforcement to piece together what happened.”

“As we continue to learn more about this tragic and sensitive situation, we are actively cooperating with law enforcement in their investigation,” reads a statement from American Airlines.

The flight was delayed in returning to Charlotte on Tuesday morning, and passengers were accommodated, Franco said. The plane is back in passenger service now.

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Write to Mahita Gajanan at mahita.gajanan@time.com.