A 64-year-old man with a gun was arrested Tuesday near the Congress buildings after he told police officers that he had a "delivery" for President Barack Obama, police officials said, dpa reported.
The man had driven up to the US Capitol to talk to the special police officers who protect Congress and ask about Obama's whereabouts so he could deliver something, Capitol Police said in a statement.
Police then questioned him and searched his vehicle. "The man admitted to having a rifle in his vehicle," police said.
Police confiscated the weapon and arrested Alfred Brock from the southern state of Louisiana on charges of possessing an unregistered firearm and ammunition.
The Capitol is about 2.5 kilometres from the presidential residence at the White House.
As the country's first black president, Obama has received a number of threats on his life that have led to several arrests.
In August, police in Denver, Colorado, arrested three people with drugs and weapons in a possible plot to shoot Obama during his acceptance speech at the Democratic nomination convention.
But US prosecutors later determined the threat was not credible, and did not charge the men with making a threat on a president's life, which carries a five-year sentence, the Denver Post reported.
One of the suspects, Tharin Gartrell, was sentenced in January to 15 days in jail for methamphetamine possession. A second man, Shawn Adolf, is still facing years in prison on federal weapons and drug charges. Nathan Johnson has pleaded guilty to illegal gun possession, the Post reported in January.