The driver was identified by county police as Thomasine Maria Smith, 47, of Laurel, Md.
Police said that after the driver had left the bus, it “apparently began to move.” She tried to engage the brake by reaching through the driver’s side window, the police said.
But, they said, Smith was struck by the vehicle and pinned against the wall.
She had left the bus during an apparent break between trips, Metro spokeswoman Caroline Laurin said. “I believe it was just a layover,” she said.
Smith was pinned between the left front of the bus and a retaining wall that is about four feet high.
A photograph showed the bus almost parallel to the wall, in an area that was striped for automobile parking.
A Montgomery County police dispatcher said the incident was reported to have happened near a Sears store in the shopping center.
The bus was assigned to the K6 route, which runs between the shopping center in the 11200 block of New Hampshire Avenue and the Fort Totten Metro station in the District.
A timetable shows that a Metrobus was scheduled to arrive at the White Oak end of its run at 3:19 p.m.
A bus was scheduled to leave for Fort Totten from Lockwood Drive, on the White Oak end of the route, at 3:30 p.m.
No passengers were on the bus at the time of the incident, Laurin said.
She said the incident was under investigation. The bus will be inspected as part of the probe, Laurin said.
From photos, the red-and-gray bus appeared to be one of the newer models in the Metro fleet, with a bicycle-carrying device protruding from its front.
County police said the vehicle was a 2011 XDE Metrobus.
After being extricated from between the bus and the wall, Smith was taken to a hospital, where she died, according to authorities.