Philadelphia police, including the bomb squad, have given the all-clear after a few tense moments at the Independence Visitor Center and adjoining garage.
Concerns about the possible existence of a bomb had closed the tourist site shortly after noon. It reopened just before 1:30 p.m.
Lt. Joseph McGarrey said the center was evacuated after a "suspicious person" dressed in camouflage fatigues and driving a vehicle loaded with many "items" - including paper and clothing - was detained for questioning.
He said the center at 6th and Market Streets was searched by police dogs and the bomb squad, and no explosives were found.
The center had been blocked off with police tape along its 6th Street side. It is across Market Street from the Liberty Bell.
Among those evacuated was Ashton Seiwell, 21, a Temple University student who works for a vendor at the center, Wheel Fun Rentals.
Police ordered him and all tourists and employees at the center outside around 12:15 p.m., Seiwell said.
"I'm not really that nervous," he said as he waited to return to work. "I'm not too worried. I'm more intrigued. I just want to know what's happening."
The investigation was a joint effort by the National Park Service, Philadelphia police and transit police.