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.