The White House in Washington was locked down for about an hour overnight as the US authorities investigated what appeared to be a smoke bomb thrown over the fence of the compound.
It is believed the device was thrown during a demonstration by the ''Occupy DC'' movement.
A Secret Service spokesman said there had been no arrests.
President Barack Obama and wife his Michelle were not at home when the incident occurred, but returned while the investigation was under way.
Members of the White House press pool were prevented from leaving for about 45 minutes before they were escorted off the grounds.
Pennsylvania Avenue in front of the building was also closed during the investigation.
