The smoke bomb was thrown while between 1,000 and 1,500 people affiliated with Occupy D.C. were protesting outside the White House.
A majority of the protesters had left the area earlier and there had been no arrests, he said.
President Barack Obama and his wife, Michelle, went out to dinner to celebrate her birthday and were not at the White House when the incident began 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 White House was also closed during the investigation.