Six-year-old Jayden Wilson, the driver's nephew, had been in critical condition Thursday morning at Yale-New Haven Hospital. He died just before 2 p.m., hospital spokesman Mark D'Antonio said.
Jayden's aunt Michelle McIntosh, 39, of Bridgeport, had stopped the minivan Wednesday morning to talk with friends in another car about where to set up their holiday picnic at Beardsley Park, police said.
After she stepped out, the van began to roll. McIntosh chased it and was dragged for a short distance, losing her shoes as she climbed in and tried to stop the vehicle, police said. It rolled 60 or 70 yards down a steep hill and into a pond, where it was submerged in 15 to 20 feet of water for up to 25 minutes.
McIntosh died along with her 2-year-old son, David McIntosh, and 3-year-old Julia Boyd.
"All of the evidence we have at this point definitely points to it as an unfortunate, horrible accident," Bridgeport police Capt. Lynn Kerwin said.
The Bridgeport police scuba team pulled the victims out and rescuers tried to revive them before they were taken to hospitals.
"I'm numb," McIntosh's husband, David, told The Associated Press earlier Thursday. "It's like I'm still waiting for them to come back."
Mechanics were evaluating the van Thursday to determine whether it was in gear, whether the emergency brake was activated and other factors, police said.
Fire Battalion Chief Ismael Pomales said people nearby tried unsuccessfully to stop the van, and that fire personnel were not able to reach those trapped inside.
"It's horrible," Pomales said. "We've had children die before. That's probably one of the toughest things for any rescue person to deal with."
Blanch Arpie, a neighbor, called the McIntoshes a close-knit family.
"It's breaking my heart," Arpie said. It's just horrible."