WASHINGTON — The White House was put under a security lockdown Saturday morning by the Secret Service after a bomb-sniffing dog alerted its handlers to examine a car not far from the presidential residence and offices, a spokesman for the agency said.

The lockdown, which started around 10:15 a.m. and forced the closing of several roads, also followed a fire at a food vendor cart just outside the White House perimeter. Secret Service officials did not say whether the two incidents were related.

Shortly after noon, the Secret Service said the vehicle had been determined not to be a threat, and the fire department had contained the smoke and fire at the vendor cart. Streets around the White House were reopened, a spokesman said.

President Obama and his family were in the White House at the time, preparing to leave for Selma, Ala., to commemorate the 50th anniversary of the Bloody Sunday march across the Edmund Pettus Bridge.