The US Capitol was briefly evacuated Wednesday after a false alarm saw police tracking an aircraft they said was a "probable threat" -- but reportedly turned out to be carrying parachutists as part of a baseball pregame show.

Police tasked with protecting the complex at the heart of US government in Washington issued an initial statement shortly after 6:30 pm (2230 GMT) saying they had ordered an evacuation as they were "tracking an aircraft that poses a probable threat."

They did not give further details.

The news became a top headline within minutes in the United States, where the memories of September 11, 2001 attacks -- which saw Al-Qaeda fly passenger jets into the World Trade Center in New York and the Pentagon in Washington -- are still poignant.

But police swiftly issued a second statement to say the order had been given "out of an abundance of caution" and that there was "no threat" to the complex.

"The aircraft no longer poses a threat to the Capitol Complex and the USCP is now preparing buildings for reentry," said the update from the US Capitol Police.

Neither the House of Representatives nor the Senate, the chambers of Congress that are located in the Capitol, were in session at the time of the scare.

There was no immediate explanation for the order, but unverified reports suggested it came after a parachute display at the nearby Washington Nationals Park, a baseball stadium, part of a pregame show for "Military Appreciation Night."

NBC's Capitol Hill correspondent Garrett Haake tweeted that he had "Just watched some people parachute down over/near the US Capitol amid an evacuation order."

NBC, citing police, said they were part of a demonstration by the Golden Knights at the stadium. The Golden Knights are the US Army's official aerial parachute demonstration team. 

"Seems they might not have told Capitol Police they'd be in the airspace. One officer here told me she saw the small plane appearing to circle before the parachuters jumped," Haake tweeted.

The stadium is roughly 1.5 miles (2.5 kilometers) from the Capitol. The Nationals were playing the Arizona Diamondbacks there Wednesday.

The US Capitol was the focal point of an actual violent attack just 15 months ago, on January 6 2021, when supporters of then-president Donald Trump stormed the building in an effort to stop certification of Joe Biden's presidential election victory.

Despite Wednesday's scare being a false alarm, lawmakers and visitors were shaken by the warning.

"We just went through a very stressful 15 minutes, but we are thankful that everyone is safe," congresswoman Teresa Leger Fernández said on Twitter.

Two young Swiss tourists visiting Washington said they were walking towards the historic white domed Capitol to take a tour when police waved them away from the structure.

"They shut the security barriers behind us. They didn't tell us why and I thought it was better not to ask," said one of the tourists, who asked not to be named.