"As part of our procedures for events of this nature TSA protocol included checking possible screening anomalies from the departing airport, checking to see if Federal Air Marshals were onboard, notification to the airline, as well [Transportation Security Administration] and [Department of Homeland Security] leadership," Sterling Payne, TSA deputy assistant administrator said in a statement.
"This is an extraordinary, unusual case," William Voss of the Flight Safety Foundation told "Good Morning America." "I can think of very few times in recent history where we have seen this type of really extreme overflight."
One of the many possibilities that investigators will look at is whether one or both pilots were sleeping.
"This is unprecedented ... in terms of being so unaware of where you are and what you're doing," former pilot and ABC News aviation consultant John Nance said. "This is virtually off the charts. The far more likely explanation is they fell asleep."
Although Nance said it is common for pilots to fall asleep on long trips, the pilots' story may hold truth, former National Transportation Safety Board investigator Greg Feith told "Good Morning America."
It's possible the pilots did not hear instructions to update their frequency and never heard the air traffic controllers' repeated calls to them, he said.
"You have to tune your ear to the call sign," Feith said. "If you're in a heated discussion, if you're distracted, you can miss that call sign."
As the event unfolded, concern rose among air traffic controllers who repeatedly attempted to establish contact, using multiple methods, the air traffic controllers union told ABC News.
Eventually, controllers asked other planes in the air to attempt to contact the Northwest plane, which proved successful, the union said.
"Regardless of what happened in that cockpit, this incident highlights some of the policy gaps that must be filled to create an air travel industry that is fair to pilots and as safe as possible for consumers," said Anne Banas, executive editor of SmarterTravel.com. "Current pilot fatigue rules are outdated and have no real relevance to the modern aviation industry -- they place an unfair burden on pilots and crew, and put passengers at unnecessary risk. Plus, the simple fact that aircrafts are carrying older cockpit recorders is very surprising, and should be remedied."
The NTSB plans to interview the crew and is reviewing the cockpit voice recorder and flight data recorder of the flight, Northwest 188 from San Diego to Minneapolis. The NTSB will be investigating whether the pilots fell asleep, along with all other possibilities.
"There wasn't any problem on board -- nothing," Andrea Allmon of San Diego told ABC affiliate KSTP Thursday. "We landed, everyone got ready to get off the plane and suddenly police were getting on the plane and telling us to sit down. They went into the cockpit, looked around and then told everyone to get off the plane."
Early in 2008, the two pilots of a go! Airlines flight from Honolulu to Hilo, Hawaii fell asleep for at least 18 minutes while in the air. The plane flew past the airport and out to sea before air traffic controllers finally were able to reach the pilots, who turned the plane around. The captain later was diagnosed with sleep apnea.