A 16-year-old who walked with his friends on thin ice over a Cobb County subdivision’s lake suffered a deadly plunge when he and another boy broke through the surface Wednesday evening.

Kennesaw’s Ellison Lake, still covered in patches of ice after a week of freezing temperatures, tempted a group of teenagers to explore its uncharacteristically icy banks around dusk. But the lake is surprisingly deep, said Sherry Rosen, a resident of Waterside at Ellison Lakes subdivision.

Ellison Lake’s depth and the day’s warmer temperatures led Rosen to suspect the ice was relatively weak by Wednesday evening. When she saw the teenagers beginning to venture onto the semi-frozen surface, she called 911.

“It must have happened really quickly because I called at 5:07 p.m. and [police] got there at like 5:16 p.m.,” she said.

Rosen had left for dinner immediately after calling for help, but when she returned less than two hours later, “There were just blue and red lights everywhere,” she said.

By that point, the two 16-year-old boys had fallen into the lake. Police officers from Cobb and Kennesaw worked together to quickly recover one of the boys, who was found hanging on a sheet of ice when officers arrived. But Cobb firefighters weren’t able to recover the second teenager from the icy water until just before 7 p.m., officials said.

Darkness and the frigid water worked against them. Firefighters used oars and pike poles to break up the ice as they searched, and drones were brought in to use thermal imaging technology, fire spokesman Nicholas Danz told reporters from the rescue scene.

The firefighters and other emergency medical personnel attempted to resuscitate the second 16-year-old and rushed him to a hospital. The teen was pronounced dead, Danz said, and he has not been publicly identified.

Four emergency responders were treated for exposure to the cold, including one who was taken to the hospital for hypothermia, the fire spokesman said. Some rescuers were in the water for more than an hour, with surface water temperature at a mere 28 degrees, according to authorities. All emergency responders have now been released from treatment.

Rosen assumed the teenagers accessed the lake, which is near Ellison Lakes Drive and Cobb Parkway, by walking over the dam – an area that she said is clearly marked with “no trespassing” signs. Rosen did not personally know any of the five or so teenagers, but in conversations with her neighbors, some speculated that they had come from a neighboring apartment complex.

“I like to think maybe calling the cops saved four lives,” Rosen told The Atlanta Journal-Constitution. “I knew in my gut it wasn’t going to end well, but I didn’t know how badly it was going to end. But yes, it’s been devastating.”