(CNN) -- Helpless. Hopeless.
That's how witnesses described feeling after three people slipped into the foamy waters of the Big Sioux River, in the middle of a Sioux Falls park.
The youngest of three -- a 6-year-old boy -- was able to escape the frigid waters alive. But the other two people -- one of them his sister -- who went after him did not survive.
"These people literally jumped in without thinking of their own safety and trying to rescue that child," Sioux Falls fire Chief Jim Sideras said.
The drama unfolded Thursday evening amidst the 123 acres of Falls Park, which is named -- like the city of Sioux Falls itself -- for the series of picturesque Big Sioux River waterfalls that run through it.
According to Sioux Falls police Chief Doug Barthel, the boy had gone into the several-feet-high foam often generated there by the river, and perhaps into the water itself.
His sister, who Barthel identified as 16-year-old Madison Wallace, at one point went in after him and ended up falling into the surging water.
A bystander, 28-year-old Lyle Eagletail, then went into the river trying to help, said the police chief. He, too, was sucked in -- but not before others at the scene did what they could to save him.
One of those who tried was Napoleon Ducheneaux.
His shirt muddy, he told CNN affiliate KSFY on Thursday how he and two others had grabbed hold of the man who'd fallen into the river.
"We had him ... but he just slipped," Ducheneaux said.
"I heard him say, 'Hold onto me, I'll hold onto you.' That was the last I heard his voice."
Matthew Krier shared Ducheneaux's sense of helplessness about not being able to do more.
"Hopeless, I felt hopeless," Krier, who watched the scene unfold from a nearby bridge, told KSFY. "I wish there was something I could have done."
Authorities from various agencies converged on the scene, their task of finding the two people complicated by zero visibility in the cold, fast-moving, debris-filled waters.
Heavy machinery was brought in to break up and remove ice from the river, so that boats and divers could get in, Sideras explained. New water heading into the river was shut off Thursday night, slowing down its flow, but the task nonetheless remained difficult.
"Everybody here is essentially eyes on the water," said the fire chief. "We're looking for something maybe as small as a knee, a patch of clothing."
During the day Friday, the rescuers found the body of the rescued boy's sister. Sideras said he talked with her father afterward.
"They are all in a state of shock," the fire chief said. "Our prayers are with them, and we're just trying to help them cope as best we can."
That left authorities with one more person to find. While there's a limited area as to where he may be, that doesn't necessarily mean his body will be easy to spot.
"It's still going to be an issue of where we're going to find him," Sideras said.