WOODBURY - The National Transportation Safety Board says it has opened a formal investigation into Monday's near-collision involving a plane carrying first lady Michelle Obama and the vice president's wife, Jill Biden.

The tower at Andrews Air Force Base aborted the landing of the first lady's plane after realizing that air traffic controllers allowed it to come too close to a massive 200-ton military cargo jet. Obama's plane was told to circle to give time to clear the runway.

Obama and Biden were returning to Washington, D.C. after an appearance on "The View" in Manhattan.

This latest incident comes after a half-dozen air traffic controllers were caught sleeping on the job.