Uniformed Secret Service agents immediately stopped the vehicle after it trailed in behind the motorcade at about 4:40 p.m. EDT.
The driver, a middle-aged man, is in police custody. District of Columbia police swept the vehicle for explosives and restored access about an hour later.
A law enforcement official says the motorcade was returning President Barack Obama's daughters to the White House.
The official said the driver has a pass for the Treasury Department, which is next door to the White House on Pennsylvania Avenue.
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