The Secret Service says two people threw something over the fence surrounding the White House complex - one person from the south side and one from the north.
The two people were being detained and questioned Thursday afternoon. Secret Service officers were ordering people in the West Wing to stay inside.
The lockdown was lifted around 4:00 p.m.
The incident follows an hourlong lockdown Tuesday after a vehicle followed a motorcade bringing President Barack Obama's daughters home through the gates. The driver was charged with unlawful entry, a misdemeanor.