Thieves with stolen usernames and passwords have broken into customer accounts at American and United airlines and in some cases booked free trips.

The airlines say the incidents happened in late December. American began notifying affected customers by email on Monday.

American says 10,000 accounts were affected and some have been frozen while the airline and customer set up new accounts. Spokeswoman Martha Thomas says the airline isn't aware of anyone booking a free trip.

But at United, a spokesman says thieves booked trips or made mileage transactions on up to three dozen accounts.

Representatives from both airlines say nobody hacked their systems and that the thieves got usernames and passwords somewhere else, and tried to use them to log into American's AAdvantage and United's MileagePlus.