The airline told CBS News Thursday evening, "The company and the FAA found that the pilot operated flight 426 in a safe manner and within the bounds of all regulations."

But FAA spokesman, Ian Gregor wrote in an email, "We are still looking into this event and have not made any determinations."

"Using the phrase 'Bingo Fuel' sets off alarms in air traffic controllers," said former FAA Associate Administrator Scott Brenner.

He told CBS News he takes that phrase to mean a pilot has less than 10 minutes of fuel left.

"When a pilot nonchalantly throws out 'Bingo Fuel' because he wants to land and cut through all the nonsense he thinks is going on is an absolute abuse of the trust between air traffic controllers and pilots."

Allegiant says its airliner was cleared by air traffic control to descend to 6,000 feet for landing, but was then told by ATC the airport was closed. The FAA told CBS News Tuesday the Fargo tower was expecting the delayed flight to divert to Grand Forks, North Dakota because of its delayed departure.

"Fargo tower talked to Allegiant's operations and was assured that the flight did have an extra 45 minutes of fuel on board, as required by FAA regulation...Allegiant's operation center told the FAA the flight had an extra 45 minutes of fuel on board. However, according to Allegiant the pilot estimated his fuel differently," said the FAA statement.

While employees on the ground attempted to reach controllers by phone, flight 426 held at 14,000 feet. After approximately 18 minutes, the airline says the pilot made the bingo fuel call because flight 426 was "now 2-3 minutes from starting to use their reserve fuel."

"Whatever misunderstanding of whether the airport was open or not, having that pilot declare an emergency landing was not appropriate," says Brenner.

The FAA investigation is continuing, and the agency could take enforcement action if deemed warranted.

"To me there is a clearly a failure here, a gap, that needs to be understood. How it happened, why it happened and how they can keep it from happening again," says Rosenker. "This is senior management doing this. It doesn't make any sense to me."