A item was detected around 4:30 pm, and an area around a cargo container on an airfield was evacuated, said Dulles International Airport spokesman Robert Yingling.
The evacuation area's perimeter included "a few" gates at concourse B of the facility, the largest international airport near the US capital, the spokesman said. Those gates were closed for four hours but then reopened, Yingling said.
The Virginia state police Explosive Ordinance Disposal unit declared an all-clear at 8:30 pm yesterday, he said.
"The boxes on the cargo pallet were found to have nothing harmful or hazardous," Yingling told AFP.
During the investigation, the airport remained open and operating, the spokesman said.
The security alert came as the United States was on a tense watch ahead of the 10th anniversary of the landmark terror strikes, the worst on US soil.
One group of 9/11 hijackers took off from Dulles International, west of the US federal capital of Washington, seized the jet -- American Airlines flight 77 -- and crashed it into the side of the Pentagon.
On Saturday, former president George W. Bush joined current Defense Secretary Leon Panetta and Bush-era defense chief Donald Rumsfeld at a Pentagon wreath-laying ceremony to honor the Pentagon workers and airline passengers who died there.
Memories remain raw of the day when Al-Qaeda hijackers slammed two passenger planes into the World Trade Center in New York city and a jet into the Pentagon, while a fourth jet crashed in Shanksville, Pennsylvania. Almost 3,000 people were killed in the attacks.