WASHINGTON -- The U.S. Capitol was briefly evacuated Wednesday evening after police identified an aircraft that they said posed "a probable threat'' -- but the plane was actually carrying members of the U.S. Army Golden Knights, who then parachuted into the Washington Nationals' baseball stadium for a pregame demonstration.

The Nationals are hosting the Arizona Diamondbacks at Nationals Park, which is located a little more than a mile away from the U.S. Capitol.

Investigators were still working to determine why the event wasn't properly coordinated with law enforcement officials in Washington, two people familiar with the matter told The Associated Press. Multiple federal agencies began scrambling officials as the plane circled overhead.

The capital region is defended by several surface-to-air missile sites, as well as military aircrews on round-the-clock alert. It did not appear that any of those systems were scrambled.

Officials believe, based on a preliminary review, the pilot might not have properly reported taking off or had appropriate clearance, the people told The AP.

The alert from the U.S. Capitol Police sent congressional staffers fleeing from the Capitol and legislative building around 6:30 p.m.

The aircraft, a twin-engine plane, took off from Joint Base Andrews in Maryland and had been circling inside heavily restricted airspace close to the Capitol when the alert was sent. Radar tracking data shows the plane, a De Havilland Twin Otter, remained clear of the prohibited airspace over the Capitol Building and other government complexes at all times. Air traffic control recordings capture the army plane coordinating its flight with the control tower at nearby Ronald Reagan Washington National Airport.

The plane landed back at Andrews around 6:50 p.m. after the parachuters descended into the middle of the field at Nationals Park.

Information from The Associated Press was included in this report.