According to the news, the pilots and flight attendants only found out about this midflight. They kept the passengers in their seats and tried their best to keep the situation calm. The flight was apparently full and according to the news, the stowaway extra passenger was only caught due to the stowaway unusual activity of moving from one bathroom to another thru out the entire flight. TSA is claiming that this was a major security breach and are currently investigating how this happened.
www.cbsnews.com

Stowaway on Delta flight caught after getting aboard plane from New York City to Paris
The woman boarded Delta Flight No. 264 from John F. Kennedy International Airport to Paris Charles de Gaulle Airport, CBS News learned. She was discovered while the plane was in midair and was taken into custody in Paris.
