TMZ reports that Carrie Fisher was mid flight when the event took place.
UPDATE 6:45: TMZ reported that "United Airlines says its crew reported Carrie as 'unresponsive' when they landed," and that she was put on a ventilator when she arrived at the hospital.
Sources to TMZ say "Carrie was on a flight from London to LAX when she went into cardiac arrest. People on board were administering CPR."
The Star Wars heroine was 15 minutes from landing when the event took place, according to the article, but as of right now, no more news has been reported.
We will keep you updated as information arises.