Jonathan Rosa, 19, was one of two men being questioned by police,
Christopher Warren said. He said Rosa surrendered to police Sunday.
Police had not released the identities of the two persons of interest. Warren said he expects both Rosa and the other man to be charged with second-degree murder.
An SUV that had been stolen about a mile away swerved into Keisha Williams, 34, and her children -- Keiearra Williams, 15, Thomas Reed, 10, and Terrence Moore, 10 -- as they sold fruit on the sidewalk to raise money for a children's playground planned by their church. The three children were killed and Williams critically injured.
Rosa was not at the wheel, Warren said, describing his client as being pressured into the carjacking by the other man. The driver swerved because of a police officer in his path and lost control of the car, which hit a tree after running over the Williams family.
A neighbor who was with Williams suffered a minor injury, while the carjacking victim, who had been forced into the back seat, was critically injured.
"He turned himself in and he is doing everything in his power to try and atone for what happened last Friday," Warren said. "Quite frankly, he's having an extremely difficult time getting over the image of that 15-year-old girl coming over the hood of the car."
Police Commissioner Charles Ramsey said investigators have physical evidence linking the two men to the carjacking, including fingerprints in the SUV. He said the carjacking victim, a real estate agent whose identity has not been released, has also been able to talk to detectives.
Family members said Williams, who has two other children, had not been told about the deaths.