The car involved in the accident then struck two other vehicles, Suffolk County police said, leading to four additional injuries -- including to the driver who struck the pedestrian.
The accident happened at about 6:25 p.m. on eastbound Horseblock Road near the intersection of Old Medford Avenue. Police said Eileen Crain, 27, was walking on the shoulder when she was struck by a 1999 Toyota Camry driven by Cindy Baggee, 24, of Bellport.
Baggee, whose two children, ages 2 years and 11 months, were in the car, then swerved into the westbound lane, police said, striking a 2006 Mitsubishi flatbed truck and a 1996 Toyota Tercel. The two children, who police said were properly secured in car seats, were not injured.
Police said Crain, the pedestrian, was seriously injured.
Police said Baggee, who was not charged in the accident, was treated at Stony Brook University Medical Center for non-life-threatening injuries.
The driver of the truck, Alberto Lemos, 22, of Farmingville, was also taken to Stony Brook with non-life-threatening injuries. His passenger, Carlos Silva, 26, of Farmingville, was taken to Brookhaven.
The driver of the Tercel, Jose Goncalves, 53, of Farmingville, was taken to Stony Brook. Both Goncalves and Silva also had non-life-threatening injuries, according to police.
The flatbed truck was operated by a Farmingville business, M.I. Concrete Inc. The truck's flatbed portion was buckled and was pushed against the cabin. At least one car had its front and back ends crumpled.
With Ellen Yan and Zachary Dowdy