The white sedan hit people standing on the side of Route 210 around 3:40 a.m., Prince George's County Police Cpl. Clinton Copeland said. A tractor-trailer that came by shortly afterward may also have struck someone on the road as it tried to avoid the crash, he said.
"It's probably one of the worst scenes I've seen," Copeland said. "This is a situation that could have been avoided, and it's a very tragic situation."
Four people were taken to hospitals, Copeland said. Their conditions weren't immediately known.
The driver of the car had been interviewed and did not appear to be seriously hurt, but a passenger was among the dead, Police Capt. Donald Frick said.
Route 210 has two lanes in each direction and traffic lights about every 150 to 200 yards in Accokeek, about 20 miles south of Washington.
Investigators were working to piece together the chain of events and had "more questions than answers," Copeland said.