Story highlights First officer found sleeping at controls of a Transavia 737, says Dutch airline

Returning from a bathroom break, the pilot couldn't immediately enter the cockpit

He eventually gained access, and the plane landed safely in Amsterdam

The incident took place in September en route from Greece, Dutch officials say

Dutch airline Transavia said it has launched an investigation after a Boeing 737 pilot was locked out of the cockpit and his first officer was later found asleep at the controls.

The incident took place in September, when the airliner was en route from Greece to the Netherlands, a top Dutch safety investigation agency said. The 737 landed safely in Amsterdam as scheduled, the airline said Wednesday.

When he returned a short time later, the pilot used an intercom to ask his first officer to open the door. There was no answer, the report said.

Eventually, the pilot alerted the crew and was able to open the door himself. That's when he found the first officer asleep, according to the report.

"It's a serious incident," said Wim van der Weegen of the Dutch Safety Board, "What makes it serious is the combination of the pilot being unable to access the cockpit and the first officer being asleep.

"By 'serious incident,' I mean the flight was in danger," he said.

The Dutch Safety Board will decide whether to open its own inquiry when the airline's investigation is finished, van der Weegen said.

Laws regarding pilot breaks during flights vary from country to country. For U.S. carriers, sleeping while at the controls is a violation of FAA regulations. Flights longer than eight hours require a relief pilot on board to take over when pilots sleep.

U.S. airlines also require a flight attendant to be in the cockpit when the pilot or first officer take bathroom breaks, in case the person flying the aircraft becomes incapacitated.